Eh, depending where ur first bus stop is. If the bus stop is
served by 851, 852 and 853 (towards Yishun), some drivers will just
skip the stop if they see either of the other services (in your
case maybe 851 or 852) arriving if they are late from their
schedule.
The 2nd one... I supposed u are talking about Lentor Ave (towards
yishun), that road is known for drivers skipping stops if there are
no passengers alighting especially if they are late from schedule.
There are 6 services all plying the same route to yishun, so they
just let another bus pick the passengers up.