Thanks for the vote of confidence (I think...).
The first part is all too obvious. Think about it like a glass of water. If you filled it to 100%, and then took the tiniest sip from it, it's now no longer 100%, but some fraction less, whether it's 99.999%, or 99.1%, or 98.9%, it is definitely no longer 100%. So the phone begins "sipping" power from the reservoir of battery power the moment the charger is disconnected.
The second part is represented in this image below, which details the charging process in 4 stages...
Stage 1, Voltage rises at constant current
Stage 2, Voltage peaks, current decreases
Stage 3, Charge terminates (this is the moment it is at 100%, but only until it starts losing power again)
Stage 4, Occasional topping charge (this starts at 90% and ends again at 100%)
It's stage 4, the occasional topping charge that shows the charge level dropping from 100% to about 90% after stage 3 ends, and when the level reaches 90%, stage 4 kicks and tops off the charge back to 100%, then the whole process resumes. So after reaching 100% and stage 3, there can be multiple stage 4 episodes before you finally remove it from charge for use. If you happen to remove it before it has reached 100% again, you will leave with less than a full charge. This is often described by people who complain that their phone was on charge "all night" but when they removed it from charge, it dropped to 90% only a very short time afterwards (such as 10 minutes later).