...standard 4 min hold!!
First of all, a B737 will never make a 4 minute hold unless you're flying very, very slowly.
This is because, unlike a light aircraft, we can't achieve a rate 1 turn while limited to 25° bank.
At a density altitude of about 7-8000 feet your TAS would be close to 250 kts if you fly 215 indicated. Each 180-turn would then require about 1 1/2 minute which totals the hold to about 5 minutes. And that is only if you really fly the 25° bank.
So the question is why the aircraft flew 2 more minutes for a complete hold.
It depends on what the hold was calculated to and what was really the case.
What altitude were you at and what altitude was the hold calculated for?
Was there a significant deviation from ISA?
Was the hold executed while still descending through a level with a different wind?
What bank angle did you actually achieve?
(I often see the 737 fly LNAV turns on departures and arrivals with a lot lesser bank angle than you would expect, even when you have entered correct speeds and altitudes, with a resulting overshoot).
The magenta line is calculated for whatever ambient conditions the computer thinks will prevail by the time of execution. Any difference from this and the aircraft will still fly the magenta line regardless of actual conditions.
EDIT: Waldo also has a very good point above. Did your hold speed change while already entered the hold?