The MIDI clock in requires a change of a fraction of a BPM before it takes notice, otherwise when driven by a computer, which have lousy jittery MIDIclock outputs, it would be audibly wobbling all the time.
Given a big change, it will ramp towards the correct value. But, because the display is in whole numbers (no decimals), it will read the nearest value, so 120.4 reads as 120, whereas 120.6 reads as 121. This can cause it to appear off by one.
