It's a bit odd, I'm not sure what reason in production or otherwise would cause this to happen (perhaps a month or so, but 6 months seems weirdly long.) Perhaps a fault in one side of the boot caused a big reproduction, so the dates became out of sync - I can only speculate.
However, I see no reason for this to be a problem in practice - if the boots are stored well, which it seems like they have been, then practically there shouldn't be any difference between the two. If they seem fine and don't look odd in any other way, then I'd say there's nothing to worry about.