This could come from several sources. I will begin with the most benign. To be sure, I would need more details, such as age of house, area of the country, paint product, sheen level, number of coats of paint.

Hopefully for you, the cause is more superficial and an appearance situation. By this I mean the sheetrock finisher will generally run a long line of mud along the nails over the studs. If the original painter did not use a primer and / or not enough paint film, what you are seeing could just be a sheen difference along a wall. More coats of good quality paint should help. A first coat of primer would be in order.

This could also be a result of the studs not being true and straight. This too would cause the studs to appear to be discolored when it is more of an appearance problem.

Last and a huge concern, during the first half of the decade, the USA suffered a shortage of sheetrock. This caused an influx of cheaper sheetrock made in China. The material which came from China has been found to be hazardous and you should contact your local HomeBuilders Association to see if there is any of the China supplied sheetrock in your area. Check online. Use a Google search for "Chinese Sheetrock"