Midstream Sample Vs. Endstream Sample?

I was preparing for my test all week, and I think I screwed up yesterday. Everything I've read on the internet says to give a "midstream sample", because metabolites accumulate in the first part of your piss, but they don't explain why the end of your stream is so bad.

Here's what happened: I smoked about an 1/8th over a 5 day period that ended 8 days before my test. Before that, I had been clean for 10 days. I was taking vitamins and creatine, and drank over a gallon of liquids before my test. I was pissing so much, that I pissed outside the lab in the parking lot, and by the time I gave my sample (3 minutes later), I had to piss again.

My piss was bright yellow, and I did pee in the toilet before peeing in the cup, but I wasn't very careful about peeing in the cup. I didn't quite put enough in the cup, and by the time I realized it, I was at the end of my stream - so I was forced to use the end of the stream, because I had no choice. In the end, my sample was probably about 85% midstream, 15% endstream. The testing lady was really friendly, and she didn't seem to care that the sample was probably not enough.

Anyone out there know why a "midstream sample" is supposed to be so important, and why you're supposed to avoid giving the end of your pee stream? I understand why metabolites would accumulate in the first part of the stream, but not the last part. Am I going to be okay? I need this for a job, and I'm going to be so pissed at myself if I fail it.