It seems odd, yes, but it isn't really "extra". It comes at the expense of either a previous or subsequent year. Below is assuming that your company has a fourteen day pay period that never deviates. If your company deviates depositing pay to avoid a holiday, then it may change the results.
This can be modeled in Excel. You need three dates, start of pay period, end of pay period, and date of check.
It's the date of check that matters. Assuming last year was a normal 26:
This year, check date will be in the last few days of December, making it #27.
Next year, Start/End of last check will be December dates, but date check received will be in the first few days of January, making it the next year's income. So #26 would be pushed to #1, meaning 25 checks next year.
I don't have access to Excel to plot this out right now.
Edited for clarity.