While I agree with most, that it would be nice if they were a snug fit in the bottom of the mold, I think it's probably best that they are the way they are. The base is slightly curved upward. This actually allows air to escape early in the process so you are less likely to have trapped bubbles in the candle. This does mean you have to shave wax off the bottom before you pop the pin out. As for the ridge, it's up to you to keep it. If centered, the ridge will be even. If you don't want it, you could shave it off, or rub the candle on a hot plate.
As for centering, and keeping it centered, I put a neodymium magnet under the mold and the pin didn't budge. It also allowed me to stick the cups to an old baking sheet covered in parchment paper. This made for tremendous stability during the pour, and a pin that stays where you put it.
So, all in all, I do like these pins. The only reason I give it 4 stars is I had to straighten all 10 pins. (they were bent to the side) I worry that any given pin might pop out while trying to straighten it. Otherwise, I'm happy with them.