I know right? Erza at full power would still be around above average Spriggan Level, Mira after recovering her magic managed to defeat another spriggan as well, so those two are spriggan level, they could not defeat Irene so she is above them. Simple as that.
Mirajane defeated Heine and Juliet, intimidated said swords with her magic power, got wrecked by Irene, was pretty much tended to by Brandish, then fought Jacob with his eyes closed.
Erza went from fighting Ajeel, helped against Neinhart and intimidated him with her magic power alone, fought Berserkers, blocked a full powered attack from Natsu and Gray simultaneously with her bare hands, then destroyed Irene's strongest enchantment with a broken leg just moments after taking Irene herself on with Wendy's help.
I'm not arguing whether Mirajane is Spriggan level as that is quite a broad tier, but repeatedly making it sound like they display the same general level of showings through the mentioning of said tier is a bit misleading, no?