For ease of use, by default LightStanza provides a single input value for reflectance and transmission values. However, the simulation engine (Radiance) actually does not take a single value as input, but rather R,G, B (red, green, blue) values. 

When you edit the single value, LS combines this with the current color of the material to set R, G, B values to pass to the simulation engine, while trying to preserve the current color of the material. This is so that edits to the reflectance/transmission properties don't significantly change the color of the material.  These R, G, B values are then re-converted back to a single percentage for display, and this is not always exactly the same as the number you typed in, depending on the desired value and the color.

Typically the changes introduced by the conversion process are only a few tenths of a percent, which will be negligible as far as simulation results are concerned.

You can also click on the "View" button at the top of the materials panel and select "Radiance Format" in order to edit the RGB reflectance/transmission values directly, instead of the simplified single value, which may give you more control.