We knitters like the rules to be clear. When we read a ball band we expect it to be true. Prepare yourself, I'm about to challenge your expectation.
I treat the ball band information purely as a starting point. It's really more of guideline not a hard rule. Yarn companies want to sell their yarns so they label them with lots of information. It may say it's fingering, DK or worsted weight. Many yarn shops arrange their yarn by weight as a way of helping out their customers and their staff make substitutions. Yarns which don't fall into specific categories are problematic. Often these yarns will have great pattern support to help sales. This does cost the yarn company more money though, as in spite of their offers of free patterns, they pay either full time staff or indie designers to create patterns for them. The cost of the patterns are buried in the cost of the yarn.
Sometimes yarn companies will push the yarn into a different category. A common example is fingering weight yarn being used for socks and gloves at one gauge and then the same yarn is used for lace at a much looser gauge. I've also used heavier yarns, knit more loosely than recommended for specific reasons. Often it's been scarves or shawls where I want more drape added to the fabric.
Two of the reasons we can't just look at a yarn and determine weight have to do with the spinning process and the loft of the fibre.
The spinning can change the yarn depending on it's relative level of tightness or looseness.
I've recently experienced this when comparing two 100 % cotton DK yarns. I was able to get gauge with both yarns but I had to use a 3.25 mm needle on one and a 4 mm needle on the other.
The yarn stats are:
105m / 50 g
22 stitches, 28 rows to 4 inches / 10 cm
Yardage to weight ratio: 2.1
230m / 125g approx.
22 stitches = 10cm
Yardage to weight ratio: 1.84
You can read about yarn ratio calculations here.
Yarn 1 is much more tightly spun than yarn 2. Yarn 1 required a smaller needle to get the same gauge.
In the case of loft, think about mohair lace weight yarns with ball band gauges of 18-25 stitches per 4 inches. According to The Craft Yarn Council that yarn fits into categories 2, 3 and 4, ranging from sport, baby, DK, light worsted to worsted.