EDIT: I figured it out - the "max points per day" was set to 10 in AltaUserPoints, causing the low payouts.
On a fresh install, when using AltaUserPoints as the points system, the slot machine is giving out points incorrectly with the default configuration (see attached screenshot).
Diamond Jackpots are giving out 3 points instead of 100, Double Bars are giving out 1 point instead of 16, Bells are giving out 6 points instead of 8, etc.