Gear and Pinion Hardness Numbers

[Carburize]

I have posted this question in the Gear and Pulley Forum but just in case there is a reader who is not a regular in that area I will also ask it here.
A recent request for quotation stipulates that in a gear set the gear MUST NOT exceed 300 BHN and the pinion 350 BHN hardness.
Is anybody aware of the possible origin of such a requirement?

 

[TVP]

It certainly seems like an odd request to me. The only thing I can think of is very low temperature performance-- trying to keep surface hardness low so that a brittle carburized layer is not produced. Another thought is perhaps to prevent steel from even being considered-- maybe someone wants these parts to be bronze...?