To withdraw n BTS from a vesting balance with vesting period T you need to accumulate n*T in coin-time.

If you have a balance of n1 that has already accumulated n1*t1 coin-time you can withdraw n1*t1/T right now (for t1 < T), or you can wait another (T-t1) and then withdraw the full balance.
If at this point the balance increases by n2, you must accumulate additional n1*(T-t1)+n2*T coin-time before you can withdraw the full n1+n2. Obviously, this takes longer than (T-t1) but shorter than T.

it's linear vesting and if you haven't had other 'less-vesting' fees, you would have ended up waiting 90 days instead of 82

Somebody has just upgraded to lifetime member through me. That was good, but I have realized that the "Days left of vesting period" jumped back up to 82 days. Why is this the case?

Why would my other vesting fees get affected just because new money came into "Vesting balance amount"?