An early mild pod from the correct form seems quite probable to me.
If it is an accidental open pollinated hybrid it would have to be with another yellow-fruited plant and that is unlikely.
Genetic events relating to gamete formation (pollen in the anthers and eggs in the ovary) and fertilization can also cause unique combinations that arise in populations, these combinations are distinct, not predictable and can occur in a low % of any population of seeds. That would make it a rare or novel phenotypic segregant of Datil, which for breeding purposes can be extremely desirable and technically still represent Datil, but would be called as "off phenotype" to indicate that it is both distinct and yet part of the natural diversity of a form. Moreover landraces are often well known to have such diversity of form and Datil could be described as a landrace itself.
And of course there is always a possibility of a mistake or confusion, mix-ups can and do occur because we are all only human after all.
I would consider deliberate mislabeling last.
It is more common than we would all like however many vendors and most growers put a lot pride and work into their products to assure that they are properly identified and of a high quality. Many will gladly replace a product that did not produce the right fruit.
Edited to add:
There is another potential source of variation that I had meant to address but forgot at the time I was posting earlier.
Transposable elements are known to occur in Capsicum and can result in off-type phenos as well and plants exhibiting distinction due to them are extremely valuable to breeding efforts... but they are fairly uncommon.