weird world of vibrato

bloomer

if i put vibrato on a unison bend and bend so that both strings are ringing at same pitch, dip the pitch and bring it back up then repeat  it sounds in tune. If instead I raise the pitch above the stationary note and bring it back down to same pitch and repeat it sounds sharp. But If I vibrato an unbent note starting at its pitch and then alternately raising it and lowering it back down to pitch  this sounds in tune. This seems odd, any ideas?