I have used the C41 Tetenal kit as well when I started doing C41. I haven't done any C41 for a while but I am pretty certain that I got longer out of the chems than the Tetenal instructions indicated. Manufacturers have to be very conservative about the life of their chems to avoid, I suspect, claims from users if the chems were to die 1 day before the specified time limit :
It may also be that manufacturers have to try and take the worse case scenario into account in terms of customers who are a "bit cavalier" about chemical husbandry.
We live in a litigious, compensation claiming culture and here's an example where it bites back when maybe manufacturers cannot risk giving us "good news" in case there is the odd occasion when someone wants to shout "liar" in a court setting.