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When you use multi-city search, you will usually end up with more than the two fare components you will typically have on a roundtrip booking. Each fare component has it's own routing rules. When you originally booked, you had two fare components -- one for LAX-FLL and one for FLL-LAX. There are LAX-FLL fares that require non-stop routing, as well as other LAX-FLL fares which allow for connections with a single hop through airports like SFO or SEA. These fares require a layover of no longer than 4 hours. The system will often price out broken fares on one-way or roundtrip searches which involve more than one fare component in each direction. For example, if the layover is longer than 4 hours on LAX-SEA-FLL, it is actually two fare components -- one for LAX-SEA and one for SEA-FLL. Rather than a single LAX-FLL fare component. You can see how many fare components are used at the "fare rules" link when pricing out tickets.

Once they cancelled your original flights (which were likely fares that required non-stop routing), the system provides more flexibility. It looks like they already went way beyond both the routing rules of your originally purchased fares, as well as the routing rules of the other published fares for this route. You are reading too much into what you can get using multi-city due to the fact that it is using more than the two fare components that you originally booked.