Another reason can be performance/memory. When you index starting at 1, you either have to add/subtract 1 internally, which sometimes but not always can be opt

Another reason can be performance/memory. When you index starting at 1, you either have to add/subtract 1 internally, which sometimes but not always can be optimized away by a smart JIT/compiler, or you have to "waste" the first element in memory, trading memory for performance. Lua always subtracts 1, while LuaJIT instead went with the latter approach.