If you have a "100 unit" light bulb, and a material that blocks 98% of the light it emits, 2 units of light are getting through. If you have a material that blocks 99% of the light, only 1 unit - half as much - is getting through.

(This is why the SPF scale is inverted/measures transmittance. SPF 50 sunscreen theoretically allows through 1/50th of the UVB radiation (or whatever wavelengths are specified by your local regulator).)