Bukkit

When using the /lag or /tps command, the plugin shows an incorrect memory usage:
#################___]/Mem: u=619M f=93M t=712M
##########__________]/Chunks: 2594/5000 (51%)
####################]/1min: TPS 20.00 avg, 20.00 abs
###################_]/1sec: TPS 19.99 avg, 20.00 abs
#################___]/VDist: 9 = 361 chunks (3 - 10)

Server is equipped with 2048MB RAM and was at the time of measure, 859MB used
Thus it should have been:
###########_________]/Mem: u=859M f=1189M t=2048M

Server is running Tekkit 2.0 (CB#1938 - 1.1 API)

The memory usage seems inconsistent - all values change every time you do the /lag or /tps command.

No it's not. It all depends in the JVM. Outside the JVM you can see that the JVM could be using 1,2,4 o 8 GB of ram, but inside the JVM some of that memory it's only reserved but not used, and some are free, thats the info about.

Total memory it's the total allocated memory for the JVM, used it's the really used memory for the server, an free, well, it's allocated memory by the jvm but not used.