Guardian armor lost over time??

1. I decided to put the guardian stacking to the test, with 4 around a single unit.
2. By right-clicking, I could see it did (20 base armor, but after start they had 180 (20+4*40))
3. However, after about 10 seconds, the right-click showed only 100 armor (20+4*20?)
4. Roughly 10 more seconds, it was down to just 20, no bonus.

If this reduction over time is intended, it definitely should be advertised.