Most infantry battalions developed their own particular MI companies as the war advanced, with the result that many volunteer companies had to replace those men serving with their battalions particular company.
It should be remembered that WO97 does not go into any real detail regarding the former and there were many reasons why a particular soldier got a particular clasp, it all depended on his duties and when he actually joined his battalions own MI, for example, if he was fit for duty, he might have actually been part of their Headquarters, he could have simply been "unfit to march" although not in hospital, he could certainly have been detached for other particular duties, both, within and without his company, whilst remaining a member of his battalion.
I really don't think it matters greatly, as long as the medal is correct, you have a clear choice on whether or not to buy it and you could always replace it in due course with another example with the western front battle clasps in the future if you wish, notwithstanding, still a good medal named to their battalions MI, if I collected to the Loyal North Lancashire (I do know a couple of very keen collectors who do!) I have to say that I'd actually be quite pleased with it.