A Rare Latin Diminutive

The Latin word sumpticulus doesn't occur in Lewis & Short, A Latin Dictionary. Here is the entry in Alexander Souter, A Glossary of Later Latin to 600 A.D. (Oxford: Oxford University Press, 1949), p. 400:

sumpticulus, ~i, small funds (CYPR. epist. 13. 7 Bayard [cf. Hartel app. crit.]; AVG. serm. 356. 10).

To Souter's examples add the version of Augustine's sermon 101 published by A. Wilmart, "Le sermon de Saint Augustin sur les prédicateurs de l'Évangile," Revue Bénédictine 42 (1930) 301-315 (at 310, from section 5):

Portamus sacculum — fatemur — quando iter agimus; sumticulos in uia portamus.

Is it by chance that all three examples come from African writers?

Don't be misled by the example in C. Munier, ed., Concilia Africae A. 345 - A. 525 (Turnhout: Brepols, 1974 = Corpus Christianorum, Series Latina, 149), p. 280 — it's simply a quotation from Augustine's sermon 356 (cited by Souter).