Frontline for cats and dogs are chemically very similar. Frontline Plus for cats contains 100g/L fipronil and 120g/L methoprene. For dogs Frontline Plus contains 100g/L fipronil and 90g/L methoprene. As such, using the dog version on a cat would result in a higher dose of fipronil and lower dose of methoprene.
Fipronil is an insecticide which disrupts insects central nervous systems, and is the portion which kills adult insects. Methoprene is an insecticide which disrupts the reproductive process, preventing larvae from growing into adults. The reason for the variation between the formulas is not clear, and there is not sufficient unbiased and non-anecdotal data to say what effects such slight mis-dosing can cause to the pets.