First, a set of Epson inks in a small format printer such as the 1400 (18 ml) can be drained in as little as 6 prints or deliver many more. It is based on the coverage area of each art file (which can be huge). Is it 100% coverage or 5%? What resolution is being used, is bi-directional turned on and what is the user's droplet weight? The numbers of prints has nothing to do with when carts go empty. This is a true example of what a consumable is and why it is difficult, if not impossible, to offer sheets per cart.
If 4 and 5 are low then 6 must be very, very close as well. What is the ink level of slot 6? I'll explain why. The Epson 1400 is unique in its cart configuration with a split head (slots 1, 2 and 3) then (slots 4, 5, and 6). As it lays down ink from each side the user can have a heavy coverage film or films that happen to be printed through slots 4,5 and 6. Odds and mathematics generally balances out the films but it is feasible to have more "tails than heads" when tossing a coin.
Finally, just like you have no idea of determining the style of prints and coverage area, the Epson maintenance particular to a user (turning on and off, power cleans, etc.) we have no idea the effects of clogging nozzles or slots that are turned on or off through the accessible multi-black setup.
In this particular users case:
Find out about slot 6 ink level.