Why acrylic needs its own feed rate math
Acrylic behaves nothing like wood or aluminum under a cutter. Run it too slow or with too fine a chip load and the heat generated by the tool has time to soften and re-melt the material behind the cutting edge, welding chips back onto the workpiece and leaving a melted, cloudy edge instead of a clean cut. Acrylic wants a lighter, more conservative chip load than most other materials to keep heat from building up.
How the number is calculated
Feed rate equals spindle RPM multiplied by the number of flutes, multiplied by the chip load per flute, using acrylic's default chip load value tuned to avoid melting and chip welding rather than a general-purpose default meant for wood or plastic sheet stock.
Practical tips
Single-flute or O-flute bits designed specifically for plastics clear chips far better than standard multi-flute end mills and are usually the better choice for acrylic, since poor chip evacuation is what causes heat to build up in the first place. Compressed air aimed at the cut helps carry heat and chips away in real time, and a genuinely dull or gummed-up bit will melt acrylic even at otherwise correct feeds and speeds, so check tool condition before assuming the math is wrong.