A new VM created with VM manager was given a maximum amount of ram as 256 megs, and an initial amount of ram of the same. The monitor shows the initial amount as being 255 megs.
Looks like a rounding error in the display. Using the command line 'virsh dominfo' command I see VMs used a fraction less RAM than the max limit (which I think is expected due to way balloon driver works) Max memory: 417792 kB Used memory: 417532 kB Converting this into MB, gives 408 MB, and 407.75 MB - looks like virt-manager summary ought to round() the MB figure, rather than truncating the digits.
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Patch included upstream now http://hg.et.redhat.com/virt/applications/virt-manager--devel?cs=92b2fc9f9201 Will be included in a future update, most likely only to F7 or later though.