Conservative GC is pretty much: look at all reachable memory, whatever looks like a pointer to a range which is allocated keeps that range allocated.

Pros: you don't have to care about type information and precise stack/structure walking.

Cons: If you have range 0x8000-0x8010 allocated, and have a variable with integer value 0x8001 somewhere in the memory, it will keep that range allocated. It doesn't matter that it's an int, not a pointer. Floats and 32-bit pointers have quite a lot of accidental collisions that way.

Conservative GCs don't know which bytes in memory are actually pointers, so they treat every word in memory as being a pointer if it looks like one. This means if you have some other value that happens to look like a pointer — in this case a float — the GC will think it's pointing to some other memory and keep that memory around even though it isn't used.

Because 1.0 is 0x3f800000, which is often right in the middle of the heap.