The problem isn't the tape measure, the problem is the user. You are using the wrong measuring device and drawing the wrong conclusions due to a misinterpretation of the measurement. A tap measure is used to measure length, not if something is level. Plus the way you are measuring will induce measuring errors.
The problem is that floors are usually not level and thus desks are not level. Some desks have legs that you can lengthen or shorten. If you make a mistake in that the desk won't be level. So it is key to measure if the floor, desk and iMac are level before you can actually say if the iMac itself is level or not. For that you need either a spirit level or a bubble level. If you are certain the iMac isn't level where everything else is than you can, if you want, use a tape measure to see where the problem is and home much deviation there is. That way you also know if a small piece of paper under the foot will do.
However, like with everything opening a topic on a forum like this won't fix any problem. Going back to the store does. Get it resolved. It is very worrying that people nowadays need a forum for something like that... What will these people do in case of a fire? Open a thread to ask wether they should call the emergency services?