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Several answers have already been provided to you by other members, more competent than I am, and FTDNA should shed more light on the issue you are having with these kits. Therefore, what follows will probably provide you with little useful information.

It indeed seems to me (almost) impossible to know the list of NMVs between two kits if it exceeds the number of 30. The PV average indicated in the block tree corresponds to the average of Private Variants of the set of kits attached to a particular haplogroup (excepted some of them if they fall into specific parts of the Y chromosome, it seems to me). These have not yet been given a name (in fact, most of them have, if you search on YBrowse based on their position on the Y chromosome - FTDNA has pre-registered these with ISOGG). However, in the list of NMVs between 2 kits, a number of mutations have already been observed elsewhere in the Y tree, and are therefore named. In the example you give to us (with the X), the NMVs with the kit above indicate a single PV and 16 NMVs already named (most starting with BY18...).

My personal experience has shown me that mutations can sometimes be deleted from the list of PVs associated with a kit, without this being reflected on the associated bigY match(es) (I don't know if this list of NMVs between 2 kits is likely to be updated). Perhaps similar circumstances could give the following situation: initial list of NMVs > 30 => deletion of certain irrelevant variants bringing this number below this threshold => refresh in the Block Tree without refresh in the list of bigY matches. .. This is just a very uncertain hypothesis.

Since the two kits you are interested in are quite old, I assume that TMRCA have been estimated for the corresponding haplogroup(s). In this case, a TMRCA rather distant from the present (~1950) should be associated with a fairly high number of NMVs between those kits. Conversely, if the number of NMVs is relatively small, the corresponding TMRCA should be more recent (however the calculation of TMRCA, with other parameters which are not all known at the moment, also takes into account STR distances, and does not in principle take into account certain NMVs).

Cheers,?

Ewenn