I think 250 is an over-estimation if you were able to fly thru the exercises without hitting any road blocks.
I’d be left to believe the 300 hours is a very rough estimation. Most (all?) certs state they have 300 hours worth of content but the content of each is different. So either all the content of each cert is scaled to “match” with 300 hours, or 300 hours is wrong, and thus an estimation.
I would not focus on the time you put in, rather I would focus on the skills you learn as that is what you really gain from FCC. Don’t worry about time, completion is not a race, or heck don’t even to focus much on the certs. It’s the knowledge and the time you spend working on the challenges and projects that really matters.