Can you check the data on the drop locations for these relics? All three are listed as having 14.29%, 14.29% drop chance for the B & C rotations of Tier 3 interception. After many, many completions of the C rotations, I have 8 M1s and zero P1s and D1s. I believe this is nearly statistically impossible at these drop rates.
FOV, your reasoning makes sense considering the Sunder helm is exclusive to Excal Umbra, and cannot be applied to Excal/Prime.
However, you're misinterpreting the purpose of the Excal category. This is a category to encompass all things Excalibur, for the name's sake, not just the normal Excalibur Warframe. We do not make categories for each variant of a frame because the Excal cat already includes variants.
Yes, the Sunder helm is Umbra exclusive, but using the Excal cat for it does not change that fact. A cat for each frame's variants just creates more cats to keep track of when not necessary.
In much the same way, we don't have an Excal Prime cat because though Excal Prime is a separate frame, he is still an Excalibur for which the Excal cat covers.