Your temperature control in your home is a bang bang controller. That means it's on or off. The temperature must drop below the setting on the temp control before the heating kicks in. Your heating system has some built in lag before the heat actually gets up to the temperature you've set in. In the winter your home cools at a faster rate because it's colder outside than in the summer so the temperature lag goes on for longer which makes the house colder.
The upshot of all of this is that your house will actually be colder at the same temp setting in the winter. Check out this example for further illustration:
You have the thermostat set to turn on the heat when it drops below 17 . . .
Summer:
17 - everything's good
16 - temp dropping slowly
15.5 - heater has kicked on and is now warming the house
16 - heating
17 - all good
This cycle of dropping to 15.5 then quickly heating to 17 will happen over and over again.
Winter
17 - everything's good
16 - temp dropping faster
14 - heater has kicked on and is now warming the house
15 - heating
16 - heating
17 - all good
This cycle of dropping to 14 and then slowly heating back to 17 will happen over and over again.
So, on average the actual temperature in your house is actually lower in the winter than the summer because of the increased speed of heat loss due to the greater temperature gradient between the inside and outside. Now, we could fix this temperature control problem by using a tuned PID controller . . . which would allow you to activate the heating before the temperature drops below your set value to keep the interior feeling warmer . . . but it's more complicated and expensive to do (and can be easily screwed up) so we don't.
Thermodynamics is fun!
I'd say in the summer it probably works more like this:
Summer:
17 - everything's good
18 - temp going up because it is slightly hotter outside
18.5 - AC kicks on
18 - getting cooler
17 - all good
So you're actually experiencing an average more around 18 in the spring or summer rather than 16 or 15 as in the winter months.
Furthermore your temperature may lag lower near the windows and depending on where your thermostat is located it can take a longer time for it to realize that the temperature has dropped. So you have both lag time between thermostat hitting the right temp and telling furnace to turn on and turning on, and between colder areas influencing your thermostat in the first place.
Plus if you are multiple stories with a single zone thermostat and air system then you may find that the top floor is generally colder/warmer in winter/summer because the bottom floor will be more insulated against the extremes than the top.
Here's where the physics comes into play a bit more, the sun doesn't heat the air, rather it heats the ground then it radiates as infrared back into the air causing a lagging effect with the Earth which is why the Winter Solstice occurs in December but January and February are colder. This is also the reason why the hottest time is in the afternoon rather than at midday. With this understanding we can also understand that South facing windows in the winter months can help WARM your house because of the solar energy coming in.
Next part is how equilibrium happens, it depends on the temperature difference between inside and outside temps, so your house will cool slower when the outside temp is closer to the inside temp. The same is true for warming the house, but a major difference is that the sun's energy can be coming in straight through your windows and therefore bypassing the ambient temperature equilibrium. Just like how it can feel 10* cooler in the shade because you aren't having the sun pounding down on you.
Here's the relevant article by Mr. Money Mustache with a lot of the math:
http://www.mrmoneymustache.com/2011/11/18/first-understand-then-destroy-your-home-heating-bill/Basically what is most likely happening is
1) you have more sunlight during the day to get into your house during the Spring, thus warming your house longer, and because the temperatures are the same the equilibrium is working slower than the energy coming in causing your house to warm up, and
2) the lag in temperature to the other extreme, averaging 18 instead of 16, so really a 2*C difference (3.6*F) rather than actually comparing 17*C, and
3) the humidity difference may be a factor