this i can actualy answer.
the reason is that the company that did suply the meat never did see the animal..... for this case the animals was slaugtered in rumania ... cut out then sold to a company in france ( at this point as beef ) the french company did resell it to the english company as beef.
what we also should take a talk about is the lenght the food we eat travels ... Romania -> france -> england -> Denmark -> Sweden -> Norway, yes ALL these coutries and likely more got horse meat from Romania and atleast in Denmark and Sweden there is several products removed from the stores.