Steve B is correct. The color is actually a baked on coating, as the fiber won't accept dye well enough to get good color.
White is the thinnest strand, because it has no coating--it's the natural color. Black is next, because it takes very little to coating to get black.
Fluorescent colors are usually the thickest, because it takes a thicker coating to get those bright, vibrant colors.
This isn't specific to just Dynaflight '97. All the materials go through the same color process.
A simple fix is to add a strand to the thinner bundle, and/or drop a strand from the thicker bundle. It won't hurt the integrity of the string.
If there's a significant difference in the strand size, you may well have gotten a mis-marked spool. BCY covers 85-90% of the market...a LOT of material is processed every day, so although rare, it does happen. If you think that could be the case, send me your mailing address and I'll get you taken care of.
Chad