On every one I've seen there are two very slightly brighter bits at the bottom where the LEDs are that illuminate the screen so it's not 100% even lighting, but I haven't seen anything quite like you mention.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Do you think you could post up a picture?
On every one I've seen there are two very slightly brighter bits at the bottom where the LEDs are that illuminate the screen so it's not 100% even lighting, but I haven't seen anything quite like you mention.