My Dell desktop while booting it indicates decreasing the system memory

Some memory is allocated to video memory.

RAM can also partially die, and some of the chips will be turned off.

Have you raised the specifications for the graphics adapter recently?
In many notebooks, especially on the cheaper side, a part of the system RAM memory is reserved for use with the graphics.
You could try to tweak the settings for your screen and see if this has any effect on the available system memory.

How much RAM and what's it in not to mention what OS is involved here?

As stated above Note Books and Desktops with Integrated Video quite often use System RAM to generate the Video, it makes the NB's/Desktops cheaper for you to buy.

Also if you have more than something like 3.25 GIG loaded and this includes Video RAM as well as System RAM and you are using a 32 Bit OS that OS can not address any more than something like 3.25 GIG but this also depends on the M'Board and other factors.

They are the easy things so BIOS may report a different Value to the running Operating System can show as being present and the type of computer also comes into play here. That means if it has Integrated or On Board Video as apposed to a dedicated Plug In Video Card with it's own GPU and Video RAM.

Col