4

Lying? No. But they are usually wrong.
You know, you honestly cannot tell how much memory does an antivirus use unless you are its developer (and even in that case, you can speak only for your own product, so you can’t really compare with others). Checking the usual programs (Task Manager, Process Monitor) gives you some numbers - but they are usually misleading. Unless you know in detail how the program is implemented (what memory is shared between the modules, etc.), you can’t say that a program is using less memory (or other resources) than another one.

The only thing you can tell is your subjective impression about your system responsiveness (with the antivirus installed, as compared to the previous state) - but that’s something that may significantly differ accros systems (i.e. what’s fast on one computer may be slow on another, and vice versa). So, my advice is: try for yourself and stay with the product that suits you the most.