In this position I played what I thought was the obvious move 9-8, 9-7. XG marked this as a huge -0.147 blunder and gave the correct move as 7-4. It was a single point game so gammons were not relevant Can anyone explain?

He has no spares on his 23 so you can crack it. The bar tends to be the best point to break in these positions since it's tough to clear later and if it's gone your sixes are killed. Right now sixes are forced from 7/1 which is quite bad.

Why volunteer a shot with 7/4 instead of 7/5 7/6 though?

I assume this is double match point, you should say the score/match length when posting a position.

Black's only realistic chance to win this is to maintain both anchors and not crunch his homeboard anymore. So if White plays 7/5 7/6, and Black gets to run from the 22 without hitting, White then has to likely break another outfield point next turn. This gives Black a reasonable chance at his goal. But if White gets hit as Black jumps with a 4, he no longer has a break another outfield point next roll, and this gives Black more time to crunch his board or be forced off the 23 by rolling 5.

If Black rolls a 5 instead of a 4, White does prefer not to be hit, but that's a smaller factor since Black is kind of dead regardless.

Many thanks for your explanation. I did wonder if it was a timing issue and it now makes sense. I did state in the original post that it was a single point game.

Glad I could help, sorry I missed the score comment.