As I understand it each bank is slightly different, but it may well be that the sum of your small transactions never went over the value requiring your PIN to be entered before you then made a large transaction where it was entered anyway and reset the count.I do lots of small transactions, typically 3-4 a day, usually under £10. Transactions over the contactless limit - maybe twice a month. Before I switched to using the phone, I used the same card for all of them, and rarely used a cashpoint (in fact never with the card I spend on). As I said, only once have I been asked to PIN a contactless payment once .
Back when I used physical cards I knew that one of my cards required the PIN to be entered after six consecutove contactless transactions. On one occasion this was useful at work because I was involved in making a staff training video demonstrating the risk of people thinking they'd paid but walking off (we had a thread on this a recently). We needed to video a transaction at a self checkout where contactless was used to pay but where the PIN had to be entered. We only needed one take using my credit card...