If this is the skill that governs how many items can be researched at one time, there are two skill increases where the number of items that can be researched does not increase but the time it takes to research is decreased. Could it be possible that you were at one of these points?

0/4 - 1 item researched, default time
1/4 - 2 items researched, 5% time reduction
2/4 - 2 items researched, 10% time reduction
3/4 - 3 items researched, 20% time reduction
4/4 - 3 items researched, 25% time reduction, limit to 30 days

Unless you are paying attention to the time remaining on a current research project, it is difficult to notice the skill increase taking place.