As hard a they work to keep lines pure, every seed company has a few  escapes.  Once it gets past the foundation stage there is a cost/benefit ratio that can't be ignored.  One seed in a bin can let an offtype trait show up in the final product.  It can come form any of several sources.  A missed rogue, a female self, stray pollen, or seed handling equipment that wasn't completely cleaned of the previous line.  It could even be a spontaneous mutation.