Puerto Rico's choice of shields for route categorization

[Pink Jazz]

Puerto Rico categorizes its routes into four different types: primary, urban primary, secondary, and tertiary.  Primary routes use white and blue shields, and urban primary routes use the same design but in black and white.

However, one thing that seems a bit strange to me is the choice of shields for secondary and tertiary routes.  For some odd reason, secondary routes use blue and gold "county" pentagons, while tertiary routes use black and white circular shields.  Doesn't anyone think this is counterintuitive?  I personally think it should be the other way around.  I wonder why would Puerto Rico go with such a weird scheme.

[codyg1985]

Maybe the route shields in PR weren't developed to to match the US route conventions, so they figured that it would make more sense to make the more colorful routes the higher ranked routes.

[oscar]

Also, many of the tertiary route shields are ovals rather than circles, to fit four digits. Seems easier to stretch a circle, than a pentagon.