Congratulations! You’re now officially a Home Brewer. Welcome to the club!
Two issues happen with gravity readings for extract batches in buckets.
Those volume lines on the bucket are inaccurate. Too late for this batch, but with any new bucket, I recommend getting a real gallon measurement, like a water jug. Mark the original level of the jug, then dump it in the bucket. Mark the “true” 1 gallon line on the outside of the bucket with a sharpie. Refill the jug to the mark you made on it, then dump it in the bucket again so you can mark the “true” 2 gallon line. Repeat for 3, 4, and 5 gallons. With bad bucket lines, you may have inadvertently added too much top-off water filling to a 5-gallon mark that is too high on the side of the bucket.
Stratification. When you mix the top-off water, it’s actually surprisingly hard to get a thorough mix. With stratification it is likely you took a sample that had more top-off water than wort. Stratification totally screws-up your OG reading. I’ve gotten readings 20 points too high or low! But aside from a bad ABV calculations there is zero impact on the fermentation or the final beer.
For your beer I suspect you have a bit of both. But the good news is, you’ll be fine.
So, have you ordered your second kit yet? The biggest mistake new brewers make is running out of homebrew.