Ram manufacturers restrict what ram modules are allowed to be used for. Some modules Framework is only allowed to use in pre-built machines while other modules Framework is only allowed to sell to customers for customers to install themselves.
With DDR5 (which is used in the current AMD based Framework laptops and the upcoming Intel Core Ultra based Framework laptops) Framework has worked out a deal with ADATA for ram modules that Framework can use in whatever way Framework wants (and has Framework’s name on the modules), but that doesn’t apply to DDR4 modules like the Intel 13th gen laptops use.
Although even with modules that Framework can use however they want that wouldn’t necessarily prevent the issue you’re seeing. To reduce handling times Framework builds the pre-built laptops ahead of time. That means that it is possible for them to end up in a situation where they run out of ram modules that are ready to put in the box for a DIY edition order but they have a bunch of already packaged (ready to sell) pre-built laptops with ram in them. In that instance it probably isn’t worth the employee labor to unbox and disassemble those already built laptops, so they will just wait for a new batch of ram modules from the ram manufacturer before it is available in DIY models.