In every laser printer I have ever used, when toner is running out, it becomes evident on the left hand side of the page before it becomes evident on the right side of the page.
The printed matter always, without fail, appears lighter on the left first.
Why?
Just to hazard a guess, it's because you're using English or other left-to-right languages. Since we read left-to-right, we (typically) start lines of text on the left, so the left side is used more heavily.
Some printing styles will mollify this a bit -- printing right-justified text, printing mostly graphics, printing right-to-left languages, or printing newspaper-style evenly-distributed columns.
This is why we used to extend the life of our toner cartridges by pulling them out and shaking them, often several times, before completely replacing them.
