Actually it doesn't really make sense. To be able to use titration, the machine has to know exactly how much was used in each test. So it should have the capability to calculate how much has been used to date and know how much reagent is left. So the 1/3 leftover reagent actually makes less sense.
You're right it doesnt make sense given the technology in use but Neptune will probably come up with something along the lines "From the get go it was designed to do that" - Wait what?
That was the answer given to similar question in a recent video.