I recommend a bit of calculating to ensure you’re milling fine enough.
From How to Brew Chap 18: Calculate how efficient your extraction was. Measure the gravity in the boiling pot and multiply the points by the number of gallons you collected. Then divide by the number of pounds of grain you used. The result should be somewhere around 30 ppg. 27 is okay, 29 is good, and over 30 is great. If it is 25 or below, you are lautering too fast or you are not getting good conversion in the mash, which could be caused by having too coarse a grist, the wrong temperature, not enough time, it got cold, or a pH factor, et cetera.
BIAB can use finely milled grain because they aren’t traditionally lautering once mash is complete or continually using vorfaul during the mash. If the liquid needs to flow thru a grain bed (as in lauter or vorlauf), it probably will have to be more course than what is typically used in BIAB.
Retailers have to take into consideration that their clients come from a spectrum of brewing techniques.