Are you 100% sure the chemicals are marked correctly? This doesn't sound TOO convincing : " I used some older sodium carbonate (anhydrous) I was given with a bunch of chemicals to make the 10% solution rather than the newer stuff that formulated the beers developer with "
Adding sodium carbonate to a weak developer most assuredly makes the developer more active.
If it is a high Ph developer, like Rodinal concentrate which was formulated with NaOH, you'll be diluting the mix and get less activity.
But in this instance I'm nearly 100% sure you or the previous owner mixed up the labels. can't find no reason for this.