Top loses memory over time? Memory leak?

[Tim]

Okay, here's the background. I thought top was displaying my memory 
stats oddly, so I ran a little test. I rebooted the pc, logged into the 
console as a normal user, and ran top. I did this at around midnight 
last night / this morning. I let it run until I got home from work, 
(6:15 today) So that's about... 18 hours. Here are my results. First, 
when I started last night.

29 processes:  1 running, 28 sleeping
CPU states:  0.0% user,  0.0% nice,  0.2% system,  6.0% interrupt, 93.8% 
idle
Mem: 8120K Active, 8372K Inact, 17M Wired, 8576K Buf, 460M Free
Swap: 980M Total, 980M Free

Now, the one from when I got home:

25 processes:  1 running, 24 sleeping
CPU states:  0.0% user,  0.0% nice,  0.4% system,  8.9% interrupt, 90.7% 
idle
Mem: 7400K Active, 249M Inact, 68M Wired, 60M Buf, 170M Free
Swap: 980M Total, 980M Free

This machine sat unattended all day. What could be causing the drastic 
drop in Free memory? Is this normal? When I actually use the machine, 
it's even worse... Almost all of my RAM goes from Free to other various 
states, Mostly Inactive, even after I've closed all the programs I've 
had open, rather quickly (maybe an hour or two)

For reference, the uname -a:

FreeBSD Empathy 5.3-STABLE FreeBSD 5.3-STABLE #0: Sun Nov  7 21:38:11 
EST 2004     Niy at Empathy:/usr/obj/usr/src/sys/SMP  i386

(Dual processor, PIII 1.0ghz, 512M Ram)