[pescetom]

Here the explanation of 3♦ is criminally reticent in terms of both strength and length of fit: is the fit "9 trump" as explained (no) or "The law" (yes, but that is 10 trump) or 2+ as stated (insert comment).
The successive explanation of 4♦ is more bashful about length but equally reticent in terms of strength.

[smerriman]

Standard case of GIB being unable to display 'or' in descriptions. 3♦ is:

3 diamonds and 4-16 total points, write law in the description (from the LOTT rule)
or
2+ diamonds and 14-17 total points (from a generic rule about making a non-jump bid at the 3 level when LHO has bid something)

Combining both descriptions gives the result.

In this case it's the second rule firing; if West passes or North has one less point, it triggers a different rule about bidding 4♦ based on LOTT.

[Huibertus]

I really don't care about the descriptions in this case. 3♦ is not what I would bid myself (i'd bid 5 hoping they judge the hand wrong, it can't be easy for them, and 5♦ could make or be a good safe too). However I do realize the tactics of bidding only 3♦, allowing partner to double 4♠ if he's got defense for a ♠ contract, could ALSO lead to succes. Such as on THIS actual hand.

Simply by not wasting ♠Ace for a useless ♦ switch, any ♦ tricks are not going anywhere, but using it to keep control by ducking, forcing declarer either to allow you to ruff a high ♥ or to lose trump control when you finally take it and remove dummy's last trump, which all in all is basic defense.

Just take your 500 or more if declarer errs after ducking ♠ Ace.

[pescetom]

Thanks for the explanation of why it happens, but it still happens.
If one tells the opponents that the agreement for 3D is "Law" based, they are entitled to assume that with 4 cards the robot would bid 4 (as is partner, if the explanation is the only available system documentation).