If the traffic is between two ports, members of bridge, then you won’t see that traffic in bridge counters. Bridge in RouterOS has a few personatlities and one of them is bridge interface which is a connection between RouterOS (the software part) and bridge (the switch-like entity). The counters you’re observing are counters for this personality and count only traffic directly handled by ROS (e.g. the management traffic).
If the traffic you’re mentioning (skype call), is being routed by ROS, then it should show in SNMP output, because traffic does traverse bridge interface on the way to/from routing engine (the LAN leg of it … assuming bridge is used to connect LAN ports together). Beware that some communication apps (e.g. skype, viber, …) use direct connections between call parties and if both parties are local, bulk of traffic never leaves local network.