3 minutes low speed is the point where your low speed points equal your high speed points. If your low is less than 3 minutes, your high speed points exceed your low speed points. Way back when, high speed was the important thing and low speed was merely a formality. The line length requirements probably go back to the original 1950 rules.

Half a foot of line length out of 60 feet is equal to half a second out of 60, or about .2 seconds on a 24 sec. hi speed. Not a lot to get hung up on!

Carrier is sort of an oddball event in the both high & low speed matter.  It's also the only one with a one-sided tolerance.

Of course in combat & racing, it's essential that the plnes have nearly equal length lines.

Speed uses a minimum only.  Obviously, you're beating yourself with long lines.

I think in the early days of Carrier they ran it like a speed event and only cared about high speed.  Somebody probably dreamed up a theory that somebody might cheat on LOW with long lines, hence they clapped on the +6".   But in reality, the smart constestant is as close the minimum as humanly possible.

In the old days, profile carrier went like this:  Everybody went about 20 MPH low and the HIGH's varied from 80 to 90 MPH, so High speed was everything.

In modern Profile Carrier, High Speed typcially varies from about 26 to 20 seconds and LOW Speed varies from 60 to 240 seconds, so the winning is done at low speed.  With the model hanging virtually stopped, ling length means almost nothing.

The basis for the carrier rule may be because it is basically a speed event. In speed there is simply a minimum length. Since carrier has both high and low speed, a maximum length is needed. So in carrier we have a minimum 60' and a max 60'6"