Please help me understand this charging algorithm
When cells are mismatched, one cell can over charge while another is undercharged. This time is called equalize. Since you are using gel cell, you don't have to be concerned about this. So after mode 2 duration, optimal battery life is regulated at float if PV can supply it.

I am not sure of the algorithm for going back to Mode 2 voltage depending on duration of voltage below Mode 3 but it is likely counting coulombs of charge by I*t and putting up to twice as much back as depleted or simply goes to float Mode 3 when supply current = load current ( 0 charge into battery) .

PV's are in theory current sources but lossy so the max power MPPT, curve peak finder hunts for this, but it is usually around 75 to 80% of the no load max voltage . If you are close to this the curve is fairly flat so no big deal, but this may depend greatly on PV chemistry.