How can a non rechargeable battery AA size 1.5v after 8 years in a calculator still have 1.5v?

i bought a scientific calculator 8 years ago and never changed the battery. i was using it since i bought it.today i checked the battery with a multimeter because it should be empty write now but it showed me 1.50v? it's a panasonic battery 1.5v aa size in a casio scientific calculator fx-95ms. i want to know how it's still 1.5v after 8 years of usage?