Lower Power GDDR4 "X" where the X is something simular to GDDR6 vs GDDR6X - they just have a pin here and there extra that provides a little bit more bandwidth then normal GDDR4/GDDR6.
I guess costs of production where the reason.
You're misunderstanding the question: to the best of my knowledge, there exists no RAM standard called LPGDDR4X. I can't even find any tangible mention of this type of RAM previous to this card. Which begs the question: where is this RAM coming from, who is making it, and who developed the standard? Developing a new memory standard, even one that only modifies an existing one, is a decidedly non-trivial task, as is having memory and memory controllers designed, tested and manufactured. So, is this an in-house thing? Is it done in collaboration with some memory manufacturer, and if so, who? What is the technological basis - GDDR4 (ugh), LPDDR4X (better!), or something else entirely?
Also, "a pin here and there that provides a ltitle bit more bandwidth than normal GDDR4/GDDR6" is such an absurdly simplistic statement that ... well, what's the point? Adding "pins" means adding I/O to both the memory die and the controller, both of which are non-trivial. And grouping GDDR4 and GDDR6 as if they have even remotely comparable performance? Even if you meant LPDDR4X rather than GDDR4, that is still a
drastically different technology than GDDR6. GDDR and LPDDR standards also have broadly different design goals - GDDR is bandwidth at all costs, even if efficiency suffers, and latency is crap; LPDDR is low power first, then latency, then bandwidth. These are not compatible technologies, and somehow mixing the two would be anything but trivial.
The simplest explanation is for "LPGDDR4X" to be a high-clocked, latency-be-damned, higher power binning of regular LPDDR4. That would work with established PCB design tools and (likely) memory controllers and memory chips too - you'd just need to bin things and ensure signal integrity and data corruption stays at acceptable levels. But given that no information about this exists, at least that I can find, this is an open question.