Buffer Capacity Glycine

I've been working on a Biochem lab paper and for the life of me cannot figure out why our onium was stabilized and had a better buffer capacity then our carboxyl.

Here's an excerpt from the assignment:

Looking at your data below, the carboxyl pKa decreased by >2 log units, this means that the carboxyl on glycine is approximately 100 times stronger an acid than the carboxyl on acetic acid. Interestingly, we predicted a weakening of the onium pKa, but the difference in pKa of the onium is very small by comparison both theoretically and experimentally. Why is this so? Theoretical Experimental Acetic Acid: 4.75 4.56 +/- 0.144 (N=15) Gly-Carboxyl 2.34 2.36 +/-0.35 (N=4) Ammonium 9.25 9.50 +/- 0.23 (N=6) Gly-Onium 9.60 9.57 +/- 0.061 (n=4)

Any help would be greatly appreciated! Not looking for the answer, just a nudge in the right direction