Here is a lengthy discussion:
http://forum.mikrotik.com/t/rc-solved-ros-wireless-feature-request-wireless-access-list/111579/1
Basically, in a wireless access-list you can specify a signal range required to establish/maintain a connection. Bugs in the client radio, bugs in the AP radio, or actual signal changes will cause a client to be disconnected from the AP when the calculated signal strength goes out of that range. This sounds good in theory, but in practice this happens for certain clients regardless of their distance to the AP (likely a bug in radio firmware.) Because the offending clients only exceed this signal range for a small number of frames at a time, the new access-list default in 6.41 is to allow a client to be out of signal range for 10 seconds before the rule is triggered.
In my experience, using a signal range of -120..120 (or not specifying a signal range at all) in the access list still results in both “too strong” and “too weak” events that disconnect clients. The “allow-signal-out-of-range” keeps these clients connected to the AP while still preventing low-signal clients from initially connecting. Unfortunately, this change didn’t get implemented in caps-man when it was put into /interface wireless access-list.