It's also the charging behaviour. The generic car battery is designed to basically stay at full charge and be able to dish out 400+ amps to crank the engine. It's fully recharged within minutes of engine startup.
A leisure battery is a deep cycle battery. Designed to run for days on low drain without an appreciable increase in internal resistance and thus without appreciable voltage drop. It's then recharged when the trailer is hooked up to the car when it's being towed.
Deep cycle batteries have more and thinner plates than a car battery as they don't have to stand up to the 400+ amps load when starting a cold engine.
They're different devices really, despite that the operating principles are identical.