just a question, shouldn't the final time be time remaining minus 90 seconds instead of just having the time remaining?

The remaining time is just easier to calculate, I mean, you don't need to calculate at all. But taking the final time and then minus 90 seconds is just a bit harder. But, both ways are correct, it's just the first one is easier, that's it. That's why we are using the final time as a leaderboard time. And by the way, if you minus 90 seconds of your final time, you will get a -time, for example: if your final time is 23sec and you minus -90 seconds, you'll get -67 seconds, and that's just weird.

First get rid of the negative and it’s the correct time. Second if I went faster and had a higher remaining time wouldn’t look a bit odd on the leaderboard?

If you get a higher time, then you'll be higher on the leaderboards. The leaderboards times are reversed, so to speak, so the higher your time is, the higher you'll be on the leaderboards.