I have 2 batteries, and even when quite discharged, the PD doesn't go to boost 25% of the time. I doubt that is controlled by voltage at all. And this isn't the first PD, or batteries I've had that does exactly the same thing. Someone said the different stages are controlled by timing more than anything else.
I would tend to agree.
I've always noticed that the charging stages keep changing, (flashing lite) even while the batteries are at 90% on solar. I don't know what keeps changing those stages, except for timing. But no matter how the PDs work, they work good.