The gemara (Eiruvin 3b) identifies two ways of measuring tefachim: tefachin sochakos, a wider measure (spread fingers), and tefachim atzvos, a narrower measure (fingers held together). When it comes to Torah law we usually adopt the stricter measure to avoid any doubt. The Minchas Chinuch asks why the poskim to do not make clear that the 10 tefach shiur required for a ma’akah built on a roof must be measured using larger tefachim l’chumra? The Emek Bracha answers that issue of sochakos/atzvos is relevant only when a specific quantity must be measured. The mitzvah of ma’akah is not to build a railing which has a specific quantity of height; the mitzvah is to build a ma’alah which has the quality of preventing someone from falling off the roof, a quality which Chazal tell us 10 tefachim is sufficient to accomplish. We don’t apply the rule of quantifying l’chumra when dealing in qualities of a mitzvah object.
The Emek Bracha uses a similar sevara to explain the shiur of shofar. The gemara tells us that a shofar must be big enough to be held in one hand with the ends seen sticking out slightly, which poskim explain is equivalent to a tefach. It seems that it suffices to measure using the smaller tefach shiur in this case, even though the mitzvah of shofar is d’oraysa. Why do we not require the stricter measure? The Emek Bracha again explains that the shiur of shofar is not a quantitative measure, but is a quality of the object, i.e. a horn slightly larger than the hand, and therefore quantitative stringencies do not apply.