Just for reference, the standard requires two measurements, one at normal operating temperature/normal humidity, and one cold condition/93% RH. The IECEE CB scheme protocol uses the terms "before" and "after" to distinguish these two measurements. So, when looking at the numbers in a test report (or talking to an IEC test engineer), "before" and "after" means "hot" and "cold". If the leakage is primarily due to EMC capacitors, the "after" readings will normally be higher than "before" due to the NTC parameter.
But you may be referring to "before" to mean a special measurement, not required by the standard, in cold condition before placing the device in the humidity chamber (i.e. one taken for reference purpose only).
In this case I would be suspicious if there is any increase due to the basic physics. Water itself is not a very good conductor, rather it aids conduction by being a good supporter of ions in solution (and the ions do the conducting). Most insulating materials only absorb trace amounts of water and do not release any ions into solution.
However, as mentioned before, some insulating materials such paper (used in some larger transformers) and powder (used in high power heaters) do become highly conductive in the presence of water. These are protected by sealing, and the humidity test is important to verify the effectiveness of sealing. If the sealing is not effective, the presence of water will normally cause catastrophic breakdown, not a gentle increase in leakage.
If leakage currents do really vary by 10% due to moisture, then more investigation seems reasonable, as it suggests some material is not only hygroscopic but also ions are becoming free, which is significant.
Consider repeating the test at 40C (as there is more than 2 times available water than at 23C) and test only neutral open condition which would also ensure repeatability as there is no power to the device (no heating) and have the highest reading to minimize set up errors. You could also test each isolating component separately and see which is causing the increase.