The first test can be affected by what you have had to eat in the previous 24 hrs, as basiclly it gives you a sugar load and sees what your body does.
Where as the 2 hr test is much more intensive.

Often people think they are being very healthy snacking on a lot of fruit when in fact it can send there sugars up

Many people have a high first test (GCT) and go onto have a normal 2nd test (GTT). So mus so that often some doctors will only order a GTT.

Good luck.