I'm not sure why some PT soap is light colored -- I'd say "brown sugar" PT soap is more the exception than the rule, because most pics I've seen of PT soap are the very dark "fudge" version.
I once thought maybe there are solids in the PT that need to be mixed up, but now I'm not so sure about that, having stirred up my PT before using it and not really finding much sediment to stir up. My best guess at this point is that the difference in color might come from how PT is made -- a not-super-precise heating process using wood that probably varies in composition. It's possible that some cans of PT might just have less of the darker colored ingredients than others due to this variability.
The percent of PT may also contribute to a difference in color. I know the popular Grandpa's Pine Tar soap is a honey color -- even lighter than Steve's. After some sleuthing, I decided this soap has 5% or less PT. That might also contribute to the lighter color for this particular product.
My PT recipes have varied over time, but all have 50% to 70% lard/tallow and 10% with the rest being coconut oil and liquid fats. Sometimes castor, sometimes not. I can't say I've seen a color difference from recipe to recipe -- they have all been dark fudge -- but I have seen a difference in softness. Less lard-tallow and more liquid fats => softer.