I assume by "5 gallons" you are referring to imperial gallons.
Reconstituted to 5 US gallons, most kits would finish too sweet.

Assuming that your hydrometer is correct and that your palate is just
very well tuned, I'd have to say that something wasn't quite right
with the fermentation and the last little bit of sugar wasn't
consumed.

Did you use the yeast included in the kit? Most kit makers include
EC-1118 for a number of reasons, chief among them though is its
tendency to ferment out all sugars. Sugar molecules in concentrated or
pasteurized juices may form bonds that are tough to break. EC-1118 is
one of the few yeasts that can reliably consume all sugars.

What temperature did you have it fermenting at? For reliable results,
kit fermentations should occur at or above 70 degrees F (21 degrees
Celcius).

You might also want to consider letting it go longer. I realize that
kits have time compressed directions, but there's no harm in letting
them bulk age longer. You may find that over time that list little bit
of sugar gets consumed by the remaing yeast.

Greg G.