Fixie Bicycle

Fixie I often used as reference for the two separate things.

Fixed Gear Bikes
and
Run Down fixative above Bikes

Why? Which is correct?

A "fixie" refers to a fixed gear bike - one with a single speed with no freewheel (no freewheel means that the pedals always turn with the rear wheel. Wheels with only one train that no longer freewheel correctly called "single speed"). "Fixie" refers to the word "fixed" which means "joint" or something that does not move or is not independent.

A "fixer-upper" refers to a bike that is old and considered obsolete in need of repair or "moderization." "Fix" in this sense means to correct, repair or upgrade.

It is possible to obtain a "fixer-upper" into a fixie, "if you see what I mean ...

OK, so after reading the "more info" you posted, after reading my response, I see no point to your question. There are two terms, because there are two different objects / things. Both are correct.