Why MDF has its own feed rate
MDF is made of compressed wood fibers and resin binder, which cuts more like a dense, abrasive composite than like solid wood or plywood. That resin content dulls cutters faster than natural wood, and MDF also produces an enormous amount of very fine dust, which makes chip evacuation and dust collection as important as the feed rate itself for getting a clean, tool-friendly cut.
How the number is calculated
Feed rate equals spindle RPM multiplied by the number of flutes, multiplied by MDF's default chip load, tuned for the material's density and abrasiveness rather than borrowed from a solid wood or plywood setting.
Practical tips
Good dust collection is not optional on MDF, both for tool life and because the fine dust it produces is worth taking seriously from a health and shop-cleanliness standpoint. MDF also telegraphs a dull tool more obviously than plywood or solid wood, showing burning and fuzzy edges quickly once a bit starts to wear, so keep an eye on edge quality as a signal that it is time to swap the cutter.