I suspect this is a disk with multipath access and the bootlist is showing the first five paths to the disk with no space left as the bootlist is only 5 devices long.
What type of disk or storage is it?
What type of system is it?
What is the firmware level of the system?
What is the AIX 5.3 TL and SP?
What type of disk driver is in use?
I'm sure I've seen this type of problem before but cannot remember the cause without some sort of trigger for my poor old memory...
Prior AIX 7 the only way to get rid of paths and devices you don't want to have in the bootlist, is that you have to delete those paths you don't want, set the bootlist and use cfgmgr afterwards to get your paths back. I don't know of another way to get this done.
Setting your bootlist with AIX 7 you can give path=<n> with the device you want to have. The path(id) can be seen with lspath where AIX 7 offers a -t switch to list those pathids directly.
You see 5 entries because the bootlist can only contain up to a maximum of 5 boot devices.