Other players deploying glider later than mine in Fortnite BR?

Objects in the terrain and the height of the terrain can affect your glider deployment height. If you dive-bomb above a tree you will deploy earlier because of the tree. Same goes for houses, mountains, cars, rocks. You want avoid these. A general rule of thumb is to avoid dropping on objects and aim for the lowest part of the ground.

Also, you kind of have to build up the game sense to when, where, and how high you should be to land at a specific place. Some good practice is to have a couple designated landing towns and chose a specific building you will always aim for when landing there. You will have done it correctly if you are the first one down

In addition to terrain height below you as @SPYBUG96 noted, the other player may have simply deployed sooner than you and glided at a more optimal horizontal or vertical angle. The steeper you glide, the faster you get lower and are able to deploy your glider. If your goal location is far away from the bus route there will an ideal time to jump at the best angle.