You're only considering the MAC level size of 64 bytes but there is also the physical layer overhead, which pushes the effective size of a packet to 84 bytes (see [1]) with a 7 bytes preamble, a 1 byte start of frame delimiter and 12 bytes of inter-packets gap. If you use 84 bytes with 20 Gbps you get the 29.76 Mpps.

[1] https://en.wikipedia.org/wiki/Ethernet_frame