ok. here is the thing. The space is there, the problem is that it is not accesible for applications.
Android systems have several partitions as explained here. Doing an df on my device I get:
Filesystem Size Used Available Use% Mounted on
tmpfs 156.5M 32.0K 156.4M 0% /dev
tmpfs 156.5M 0 156.5M 0% /mnt/asec
tmpfs 156.5M 0 156.5M 0% /mnt/obb
/dev/block/mtdblock3 250.0M 119.0M 131.0M 48% /system
/dev/block/mtdblock5 175.1M 157.2M 18.0M 90% /data
/dev/block/mtdblock4 40.0M 1.3M 38.7M 3% /cache
/dev/block/vold/179:1 3.7G 2.8G 952.5M 75% /mnt/sdcard
the space I see in the app manager is the same allocated for /data, the rest of the space is used for the cache and the android system (in the /system and /cache partitions).
A solution would be to find out how to resize the partitions in the internal memory of the phone (there is a procedure here). Another solution (which seems easier) is to put they regularly used apps in the /system partition along with the default applications so that all memory is used (root is needed)