1. Springs don't wear the way a bearing does, but the metal is tempered to achieve its springiness in a way which does often result in a gradual loss of strength over the years as the internal molecular structure changes, so they sag a bit. I'm sure real engineers will quibble about the way I stated that, but those of us who are old enough to remember the big GM cars of the '50s surely can recall how Oldsmobiles and Buicks used to get an inch or so lower by the time they were 5 years old -- Ford fans like my Dad always blamed the coil springs and insisted that Ford should have stuck with leaf springs at both ends, but he mistook Ford's superior metalurgy for something in the nature of coils.

Of course, wear in the bushings and rubber spring shims can cause a slight lowering, too.

2. Gas-pressure shocks do support some weight, whereas the old style don't, so a car with gas shocks will lower slightly as the gas leaks away, even though the fluid damper effectiveness of the shock has not yet changed noticeably.