Two possibilities occur to me: either the gauge is inaccurate at low pressures or the filter cartridge is worn out, allowing water to flow through it too easily until a coating of material has built up. That's what I figure, at least. I ordered a new cartridge yesterday, so I'll know soon if the cartridge is the reason for this. I should probably just go ahead and replace the gauge as well. We had the filter and pump assembly replaced about four years ago and, for some strange reason, the installer scavenged the gauge off the old filter, installing it on the new one. So the gauge is really old -- maybe 30 years or so? It's been accurate enough for my purposes, so I haven't worried about it. But I guess it would be good to know what the actual clean-filter pressure is.
I'm brushing the sides mostly. I do some brushing of the bottom as like continued strokes from the sides as I continue to push down then out on the brush. But my little pool vac is doing such a good job of vacuuming the bottom that there's really nothing left on the bottom right now -- nothing obvious, at least. It also climbs the sides and does them as well.