Memory is tricky. For one, there are multiple types of human memory, not just one. Some of these are the same things, just defined differently: short-term memory, working memory, sensory memory, long-term memory, episodic or narrative memory, traumatic memory, declarative memory, and procedural memory. Memories are stored differently depending on what they are. Procedural or implicit memory is stored one place, narrative memory stored in multiple locations, working memory in another place, and traumatic memory is kept in yet another location in the brain.
I'm reminded of my Headstart computer. It had a unique motherboard, designed to be easily upgradable by jacking new modules into it. The basic model was a 286. Jack in a module, it became a 386SX. Jack in another module, it became a 386DX. Swap the 386 module out for a 486DX. The maximum RAM could be exceeded by inserting yet another module. But it didn't stop there, because it also had the usual expansion slots. Add a network card, add a modem card, add even more memory. Once everything was upgraded, it wasn't the fastest machine on the block. Everything was on a separate part of the motherboard, so the bus had to work overtime, which chopped into the system's overall efficiency and speed. But it worked, and it could work from the start, just like our brains.
I suspect this is the reason why we have so many different types of memory in so many different locations of the brain. Our brains couldn't lie around waiting to evolve into what we have now, they had to function in the meantime. We're so redundant inside our heads, a teenager can lose half of her brain and eventually return to full functionality. Doesn't work so well after about twenty, the brain's decided on what it wants to look like by then.