greasy brakes on 97 Suburban???

[sixto]

I checked the rear brakes of the 97 Suburban and was surprised to find grease on the friction surfaces:





How does grease get to the pads? The wheel cylinder has some grime but it doesn't look like a major leak. The axle seal looks intact as well:



Wouldn't there be more of a mess if brake or diff fluid were leaking into the drum? What the heck is going on?

[edit] The other side is fine.

Thanks,
Sixto
87 300D