Most any hydraulic cylinder has a "wiper seal" whether a combination seal or a separate seal where the rod slides in and out of the barrel ... I'll have to disagree with a previous poster as to where your contaminates came from. I work with hydraulics daily ... if we had to stop to wipe down every cylinder upon each stroke ... well you get the idea. Imagine a farmer stopping to wipe off the cylinder on his plow every third round.
There's nothing wrong with swiping the rods from time to time, but it's by no means a requirement unless you're parked at some ocean front property. The cylinders are cheaply made ... I've seen contaminates in the barrel of brand new cylinders left over from the manufacturing process. I agree with another poster about the grease blob on the magnet ... thousands of small metal particles combined with oil seemingly look like a grease blob.