EDIT: I see you took out the line asking why I'm defending this webapp. I'll go ahead and leave the following paragraph though, which is my response to that:
I enjoy the tool a lot and appreciate that the OP was kind enough to make and share it. Beyond that, I have no affiliation, but I do feel a need to help someone who has a misunderstanding so they can hopefully get something out of it or learn something. If it's not helping you, feel free to move on, but I'll go ahead and still post my explanation in the hopes that it helps someone else that's confused like you are.
The reason why it changes the way it does is because you aren't adjusting your savings ratio appropriately, as discussed above.
Let's say you have a 1MM portfolio, and you make 100,000, and your savings rate is 20%. That means you are spending 80k per year (saving 20). You need a portfolio that puts off 80k per year. At a 4% SWR, 1MM only puts out 40k. At your current 20% savings rate, it'll take you 11 years to retire.
If you up your income without changing the savings rate, your expenses go up too! So say you up the income to 200k. Now if you are still only saving 20%, it'll take you longer to retire, because though you're saving 40k/yr now, your expenses are 160k instead of 80k, so you need a bigger portfolio to support that, so it'll take you 20.1 years to get there.
If your expenses stay the same when your income went up, then you need to move the mouse to your new savings rate. So if you went from 100k to 200k but still only spend 80k, your savings rate goes from 20% to 60% (120k saved of 200k income) and your time to retirement drops from 11 years to 5.3 years!
The problem is you aren't adjusting your savings rate to show you are saving more of that increased income.
Let me know if that still isn't making sense, and I'm sure we can hash through it together, but I'm sure you know that just because something doesn't click right away or isn't 100% intuitive doesn't mean it has no value, but in fact it may be worth taking the time to understand.
(Note to OP: this confusion is why I advocated above for a expenses input box.. then if one changes the income or expenses, it'll calculate the new savings rate for you and the new time to retirement.)