Ok, here's the thing.. there really is no anion gap. We pretend there is because the things we routinely measure leave something to be desired on the anion side.
There have been various adjustments to the way we measure an anion gap.
When technology changed from flame photometry to ion-selective electrodes, the normal anion gap went from 16 to 12 (1). It is pretty well accepted now to calculate AG using:
Anion Gap = Sodium - (Chloride+Bicarb)
Sam put out a killer blog a few days ago and this is a follow-up conversation. Check out the blog here.
References
(1) Sadjadi, S. A., Manalo, R., Jaipaul, N., & McMillan, J. (2013). Ion-selective electrode and anion gap range: What should the anion gap be?. International journal of nephrology and renovascular disease, 6, 101–105. https://doi.org/10.2147/IJNRD.S44689