Schedule scan only does a quick scan and the full scan is actually not a full but only scans files that can carry malware not all files.
This an explanation from the Program manager
McAfee Product Manager, here. I'll take a crack at explaining how we've changed the scanning technology & why. But let me first encourage those of you who like the ability to scan a particular file or drive that we are currently working on re-adding that to the product.
McAfee has responded to feedback from users who wanted to be protected, while not paying a heavy Performance price. One way to accomplish this was by shifting to a more Cloud-reliant Next Gen AV engine. Instead of carrying nearly 200MB of DATs (virus signatures) on the PC, we are trimming down DATs to about 1/5 the size and offloaded much of what was previously handled by the PC locally, to the Cloud. Instead of using only signatures & static file attributes, which have to be updated constantly, we are also looking at file behaviors (Dynamic attributes). The end result is detection that is always getting better through Machine Learning (instead of having to try to keep up by logging signatures of known viruses constantly).
Why are scans not looking at as many files?
With previous technology, McAfee would scan every file, every time - unless scans are customized. On a PC loaded with a lot of files, this could lead to a significantly long scan time, and depending upon your PC power, some perceivable lag. With the Next Gen AV, we focus on scripts and anything executable, and those file types which are potentially subject to infection. But file types, like some media which are not a risk, are not included in the On Demand Scans. It doesn't mean they're ignored. Upon access of any file, McAfee is checking for suspicious behaviors - and if seen, the activity is stopped and the file quarantined. And because McAfee is recording any bad behaviors, the product assists in rolling back to previous state.
There is never a perfect approach to Security, but with Next Gen AV we think we have addressed Performance concerns while also improving detection capabilities...kind of the holy grail of AV.
Having said all this, feel free to let me know your concerns. We design our products for you, so tell me what you want.