The basics:

I'm running version 1.1 of Skyrim
I'm only using texture mods and a 4gb patch
I've never experienced this anomaly before. It started today.

The anomaly:

Yesterday I found an orcish sword; since it dealt 13 points of damage, I decided to get rid of my steel sword, which only dealt 11.
Today I was doing a small quest that lead me to a cave with plenty of draugr and a necromancer. I reloaded quite a few times. At some point, I went to the menu and realized that my orcish sword damage was 10 (instead of 13). Other weapons suffered from this damage reduction as well. The axe of whiterun for example, had a previous damage 20 and now it only has a mere 15.

I decided to reload a previous save and the anomaly persisted. I actually had to go to a save from yesterday to get back to the original values. This made me wonder if I did anything special today but that doesn't seem the case.

Has anybody experienced this inconsistency before?

Check one of your 'bad' saves, and see if you have a disease.

A disease!
I have OF COURSE checked for such a simple...
...
...
*uck! No, I didn't check for that.

*checks*

"Rockjoints: you're 25% less effective with melee weapons"

wonderful! Now the damage decrease makes sense (and I feel stupid for thinking of bugs when the game was working perfectly fine).
Thanks for your help HeyYou. Next time I will look at "active effects" before posting.

Gotta love the easy ones. Glad it worked for ya.