I was recently hired for a small retail gig while trying to make my way through college. I asked for $X amount on my application, and after I'd already been hired, my new manager said something along the lines of, "Oh, and I noticed you wanted $X." And they started me about a dollar and a half below what I asked, saying that was the amount all new employees start at. I agreed, especially after he promised I would get pay increases steadily.
A couple of days later, I was chatting with another employee who had already been there about a month, and she mentioned that they started her off at the same amount I had originally asked for. And not to sound ungrateful, but my position is far more demanding than hers. So I'm kind of upset I'm not getting the pay I asked for, when others had. Should I do anything, or just deal and hope for that raise soon?
I wrote back to this writer to clarify, asking if he had accepted the job without confirming salary. He replied: "I did. Their hiring process was kind of sloppy, I never really had a chance to bring it up between the interview and my first day."
So here's where I'm going to chastise you. I don't buy that there was no chance to bring it up between the interview and your first day. At some point they offered you a position, right? That's when you bring up pay if they don't. You just ask straightforwardly: "What is the pay rate?" (And even if they never made a formal offer and instead just called you to schedule your first day, which can happen in retail, you just need to be assertive: "Before putting me on the schedule, we need to talk about pay.")
As you've now found out, you can't assume they're going to pay you the desired rate you put on your application! You have to ask.
And you need to have this discussion before you accept the job, because that's when your negotiating power is at its highest. At that point, they don't know if you're willing to accept the job or not and they have more motivation to negotiate with you than after you're already working there and have shown you'll accept the lower rate.
As for your coworker, people have different rates of pay for all sorts of reasons -- including because they negotiated at the time of hire.
You can't really be upset that they offered you what they say is their standard new employee rate when you didn't take the initiative to go after more. All you can really do at this point is to do a kick-ass job so that you can justify asking for a raise down the road.
And next time, make sure you do your salary negotiating before accepting the offer.