Why can I receive ethernet frames bigger than my current MTU size?

MTU is the maximum packet size that can be sent through an interface. MTU doesn't limit the maximum packet size that can be received.

RFC 1191 says:

... When one IP host has a large amount of data to send to another host, the data is transmitted as a series of IP datagrams. It is usually preferable that these datagrams be of the largest size that does not require fragmentation anywhere along the path from the source to the destination. (For the case against fragmentation, see [5].) This datagram size is referred to as the Path MTU (PMTU), and it is equal to the minimum of the MTUs of each hop in the path. A shortcoming of the current Internet protocol suite is the lack of a standard mechanism for a host to discover the PMTU of an arbitrary path.

      Note: The Path MTU is what in [1] is called the "Effective MTU
      for sending" (EMTU_S).  A PMTU is associated with a path,
      which is a particular combination of IP source and destination
      address and perhaps a Type-of-service (TOS)...