procps vs. procps-ng
You might of noticed that on CentOS 5 & 6 as well as RHEL 5 & 6 that top -M appears to work. This is because those distros ship with the original version of procps. The project was forked and there is now another project procps-ng .
Some of the details as to why there was fork, from the Fedora Project's page .
Old (legacy) procps tools had no updates for several years and that led to a massive code split caused by a local-only application of distribution specific patches, which were not merged upstream. The project became hardly maintainable since some of the newly written patches were incompatible with sources maintained by other distributors. A similar incompatibility could be noticed in the applications behavior and their command line switches. This inevitable update can be
understood as an effort to unify the procps tools across all Linux distributions.
So to be clear, the forked project, procps-ng is what Debian, Fedora, Ubuntu, and other distros are using, the legacy project, which does support top -M is still in use of several of the longer term releases that don't keep up with the latest and greatest.
NOTE: I downloaded the latest version of procps-ng. "procps-ng version 188.8.131.52-14ef" and it too was lacking the -M switch.
In running free with switches you can see the most likely reason as to why the lack of units feature is missing from procps-ng 's implementation of top .
Rounding becomes problematic, so I believe, procps 's implementation avoids the issue by not offering the ability.
Does an OK job of showing aggregate memory usage.