It seems the statistic computed for the RTCP reports uses the RTP header size, and according to the RFC it should not.
RFC 1889 (page 24) The total number of payload octets (i.e., not including header or padding) transmitted in RTP data packets by the sender since starting transmission up until the time this SR packet was generated. The count is reset if the sender changes its SSRC identifier. This field can be used to estimate the average payload data rate.
rtp_session_sendm_with_ts() function should probably be changed from ortp_global_stats.sent += packsize; stream->stats.sent += packsize;
to something like that ortp_global_stats.sent += (packsize - RTP_FIXED_HEADER_SIZE); stream->stats.sent += (packsize - RTP_FIXED_HEADER_SIZE);