The Airheads White Mystery flavor changes from batch to batch. When a run of one flavor is finished, the company may have some candy left over but not enough to produce a full run. It mixes the leftovers from multiple batches and doesn't add food coloring to produce White Mystery.
The changing flavor of White Mystery is the primary reason the composition of the candy is a hotly debated topic. Each may be a mix of flavors, or the individual bar may be one easily identifiable flavor, leading to suggestions that it is "white cherry" or "white apple" flavored.
A similar technique is used for Dum Dum Pops. When one batch finishes, its flavors will often mix into the beginning of the next batch. The company simply packages the dual-flavor pops as "mystery" flavored.