Triple crank, extra click while downshifting
Some shifters have an extra click that is used to 'trim' the front derailleur as the chain moves across the cassette.  This is to eliminate the noise of the chain gently rubbing the plates of the FD.

This data sheet for an Ultegra 6700 shifter has a section to explain this feature:
https://si.shimano.com/pdfs/si/SI-6SC0A-002-ENG.pdf