As far as I know, 965 and 161 take different routes between
Sengkang Int and Punggol Rd. 965 goes into Yishun but 161 does not.
At the same time, 965 travel along WDL Ave 7 while 161 does not.
There is some sort of competition for commuters who travel between
Sengkang and Yishun, but overall, 965 is the only bus from WDL Ave
7 and Yishun to Sengkang and 161 is the only bus from HGN to WDL.
They serve different market. As for 168-969, 969-39, I am not very
sure about the route they take.