Why does one pressure tank on my well only pressurize partly?
You have an abnormally small differential (20/40 or 30/50 are more typical, for a 20 PSI differential rather than a 10 psi differential) which will impact storage on all tanks.
Referring to a tank chart for a 62 gallon bladder type tank, the 20/40 drawdown is about 22.6 gallons and the 30/50 drawdown is about 19.2 gallons, so 10 gallons of drawdown at 30/40 seems perfectly normal - i.e. nothing was wrong with that figure for that size tank at that pressure differential.
All that you achieve by lowering the precharge to 10PSI is a big slug of water that sits in the tank and does nothing for you, since the tank still starts refilling at 30 PSI. You may also stress the bladder since you are running a bigger slug of water in the bladder at 40 PSI than you would be with a 28 PSI precharge. You'll still only move 10 gallons of that water each pump cycle, but now you'll have (probably 20+ gallons, starting from 10 PSI and goingup 30 PSI is not a standard tank chart figure) that just stays in the tank all the time - so instead of the bladder going from near empty to just over 10 gallons, it will go from (guesstimated) 20+ to 30+ gallons.