Ovos caipira

It beats me why it is that many people (most maybe based on the evidence of my ears) say ovos caipira (free range eggs in Portuguese) and not caipiras. Caipira is an adjective that is invariable in gender, since it ends in -a, unlike most adjectives that end in -o, and not in number and should agree with ovos: ovos caipiras. Egg producers also write ovos caipira on their boxes. Luckily ovos caipiras is not so uncommon on the web, 1030 hits on Google vs. 1060 hits for ovos caipira.