Tamagotchi IDL yellowed screen

So...I've had my tamagotchi Idl for years. It's in great condition cause I am very careful with it. (minimum to no scratches e.t.c)

But I recently got a tamagotchi IDL 15th anniversary that although it's beat up on the outside is practically new and unused. Comparing  the two I couldn't help but notice when I am on a white menu screen my old IDL has warmer pixels...it's not as white and bright as the other one. Now...I have some experience in tamas and the brightness is the same for both. also there is no dirt on the screen. Is this okay? Is it normal? Has it happened to any of your color tamagotchis? 

If this is what I think it is, this is probably just a result of Bandai using displays that originate from different manufacturers.

The same has been spotted with various other handheld devices over the years - if it's that, it's perfectly normal.