Why do we assume Tekhelet came from this snail opposed to the other dye producing snails?

Why do we assume Tekhelet came from this snail opposed to the other dye producing snails?

There are only 3 dye producing snails in the Mediterranean: Murex trunculus, Murex brandaris (almost identical to trunculus), and Thaïs haemastoma (a distant relative). Technically speaking one can produce blues and purples from any of them and it seems that they are all “kosher” for tekhelet production. One piece of evidence which points in the direction of different snails being used for different dyes is the archaeological find at Sidon where a football field size mound of trunculus shells was found and off at a distance another football field size mound of brandaris and haemastoma was found. This led archaeologist Alexander Dedikind (and Rav Herzog who writes about the find) to infer that separation was being done for different color production. There is a theory that the trunculus contains naturally more indigo than dibromoindigo as opposed to the other two snails which have more dibromoindigo than indigo and as such the trunculus tends to blue more naturally. But this is conjecture and, again, technically, one can produce blue from any one by simply exposing the dye to sunlight. – Mois Navon