The replica set monitor in older versions of the server initializes it's random state via:

replica_set_monitor.cpp:1010

SetState::SetState(...) :
    ...
    rand(int64_t(time(0))),
    ...

which has the unfortunate side effect of being strongly synchronized with the system clock. This can cause non-random patterns of access across a mongos / series of mongos' in cases where all of the nodes are restarted at the same time and work is distributed evenly across the various mongos'.

Switching to use a real source of randomness for seeding the prngs in each set state should improve the real randomness of later server selection

Note: It appears this was fixed in 4.2 as part of unrelated code changes