Maybe he didn't want less? And maybe he's cheap.
Fact is he's one of a number of players that would be out of contract next summer. So rather than need to replace 2 mids in Winks and Soumare, 2 fullbacks in Ricardo and Thomas, 2 forwards in Daka and Ayew, we have one less thing to think about next summer. We could be in trouble if we just let them all run down their contracts and Thomas is probably one of the easier and cheapest to sort.
We're going to have a big headache next year just keeping a squad together, so this is a simple fix for a small potential problem.