Can anyone explain why the suggested move is so much better than mine and mine is close to a blunder?

After your move you have two builders on the 4 whereas after his move he has one builder on the 5 and one on the 4. So he will have one chance more to close.

There is value in contiguousness. For instance, you'd rather have position 1 then position 2 here:





And I'm sure you see the reason -- if White comes to the edge of the prime with a 1 (position 1) or a 2 (position 2), he'll have more numbers to jump the prime in position 2.

So in a position like yours, it is conceivable to prefer the contiguous position (your play) to the one with more builders (the computer's play). But the fact that own the 9 point counteracts the contiguousness argument. So if the opponent rolls a number that brings him to edge of the prime, he has the same chance of jumping the prime next roll after either play. This is true even though your play is more contiguous. So you should prefer the builders in this case.

Note the 9 and the 3 points are 6 apart from one another which should give you pause before making the 3. It's like the 9 point is redundant with the 3 point because they don't form two points in the same 6 prime.

Here with the 10 point made instead of the 9, "your play" becomes correct.

Thank you.