I was wondering if it usually / ever works?

I saw a cheaper ticket on budgetair but still decided to buy the same flight through BA's website. The price difference was GBP 70 and it was just a cheapo non-refundable economy class ticket in Europe.

BA came back to me and said that the cheaper ticket on budgetair was a different fare class (S and S, whereas the one I got through BA's website was S and M, respectively) so I am not eligible to get a refund of the difference in price.

This would mean that it's extremely unlikely that the Price Promise will ever apply -- usually the reason the same ticket is cheaper on another website is that a cheaper fare bucket is available (don't know how come it was available on budgetair and not on ba.com).

The interesting thing here is that (1) you can't see the fare class when booking a BA ticket on ba.com -- you only get a generic description during the booking process, so how would you know you're booking something different, (2) the Price Promise says it should be the same type of ticket (e.g. flexible, non-refundable), but not that the fare type has to match exactly, (3) I compared the fare rules for S and M classes for my ticket and they are exactly the same -- not a word differs! (I know there must be some differences -- but they're not included in the version available to public).

Any one had good or bad experience with this?