Currently mbpsSendRate and mbpsRecvRate are calculated based on the interval between srt_bstats(..) calls with clear argument set to true. The argument tells whether to reset interval-based stats like byteSent, or not.

double interval = count_microseconds(currtime - m_stats.tsLastSampleTime);
perf->mbpsSendRate = double(perf->byteSent) * 8.0 / interval;
perf->mbpsRecvRate = double(perf->byteRecv) * 8.0 / interval;

If interval-based stats are not reset, the bitrate calculation interval also grows, making mbpsXSendRate averaged over a constantly increasing range.

Consider using some averaging over one second. For example, storing a moving window of ten byteSentTotal values on every 100 ms, then calculate the rate over the last second using edge values.
Might be worth smoothing this value additionally using a moving average.

SRT Version: 1.4.2 and prior