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Actually, in the early game it is usually better to play 13/7 as in the left-hand position (rather than 24/18 as in the right-hand position) with the 6 when the other half of the roll is hitting inside. I more perceive 43S-61X (due to Opp's 10pt builder) as an exception to the H rule than the other way around.
The usual reason for coming down with the 6 is that it typically gives Opp some non-hitting aces. For example, with 54S-63 or 54S-61 (though for the latter position P is better than either), H is better than X because Opp is denied hitting back with 31 21 11 (there is no blot on her 7pt).
This position (i.e., 43U-61 diagrammed above) is a little different. Opp already has good aces (21/20*). However, Opp does NOT have good 6s (except for 61). There is a big swing on 65 (which fails to hit a second checker), and 64 63 62 (which fail to hit any checker at all), and a moderate swing on the 66 fan (after which Blue can use his 7pt builder to attack).
Against that, H gives White more double-hit rolls, but 11 doesn't matter (White won't hit twice) and 51 hardly matters (because anchoring is not much worse than hitting a second checker). On the other side 65 is already been accounted for in the bad 6s. Countably, then, the double-hit difference is 52 53 43 minus 55, which is 5 numbers. Also, 61 33 hit more profitably, and White's non-hits 42 22 are somewhat favorable to Blue for having started the 18pt anchor). But all that does not quite add up to the high cost of bad 6s explained in the previous paragraph.
I am curious if someone can come up with a plausible argument for why 43U-61H is right for money but 43U-61X is right at DMP.
Keep in mind that for money H is not a LOT better than X. The Snowie-Gnu agg margin is only .013. And at DMP, the margin favoring X is only .003 (and it's only 5k, so the verdict is still out). There is not much distance to travel.
That said, I believe the main reason that X becomes better (or at least closer) at DMP is that anchoring on the 18pt is highly efficient at that score. Owning that anchor yields good winning chances, while posing a smaller threat of gammons (that don't pay extra). At the same time in a sense the 18pt is more gammonable (which doesn't cost extra) than other interior anchors that Blue might otherwise make (where he can't get a checker closed out). This is the same conclusion I came to when I created the four-score position that you posted here (where the best play at DMP and at no other score is making the 18pt). It is true that H gets gammoned more often due to the 5 double hits (a risk one wants to take at DMP), but that is largely offset by the extra gammons H will win due to White's 62 63 airballs.
Hope that helps,
Nack