I am a hijabi.
Unfortunately these days, many people don't follow a religion because they want to but rather because they are either pressured into following or forced to.
There are many reasons for what you see and tbh it's a common sight, everyone either knows a hijabi in a pre marital relationship or have seen one or have heard of one.
Like another poster said, it comes down to brought up and the person themself. You either get girls who willingly wear the hijab for religious purposes or you get girls who are forced into it by family/pressure or you get girls who wear it to seem religious. Now depending on what category the girl falls into and what her intentions were behind wearing the hijab, she will act accordingly (this ofcourse doesnt apply to everyone as no one is perfect and humans make mistakes); if a girl is to fall into the later two categories, she probably wouldnt be against making out or engaging in pre marital relationships. It is also down to faith and how much you believe and how strong you are. I Have hijabi friends in relationships with guys and even though they are extremely devout, and they know what what they are doing is religiously wrong, they are humans, have made a mistake and cant get out ouf it/dont want to.
You also have to bare in mind that it isnt just hypocrisy for the muslim hijabis but rather its hypocritical for anyone who calls themselves a muslim.
It all comes down to the girls faith (iman), intentions and brought up. If she only wears it because her parents forced her to and not because she believes, then it shouldnt be surprising if you see her making out with a guy.