Do teaching load agreements always refer to the academic year? I was lucky enough to get hired as a tenure-track assistant professor in computer science. During the negotiation of the offer (per email), the department head assured me that, during my first year, the usual teaching load of 2 courses per year will be reduced to 1 course. 

After having accepted the offer and agreeing on a starting date in Spring (2017), I was told that I will be teaching 1 course in Spring (2017) and also another course in Fall (2017) as "I am not teaching any course this Fall (2016)" and hence I have a reduced teaching load as promised. The reasoning makes little sense to me as I will only be joining in Spring 2017. (For the record, I am going to be TA for a course at my current institution this Fall).


How do I approach the situation? 
Is it common that new hires joining in Spring miss out on reduced responsibilities during the first year? 


Note that the department/university is considered to be well-established (i.e. reasonably highly ranked and has been around for several decades.)

Edit:   I will also be teaching a course in Spring 2018.
I've seen this sort of thing happen from time to time; these situations are sufficiently open to interpretation that if you don't have clarity in the letter itself, the chair may try to get you to do a bit of extra teaching. It's entirely reasonable to push back a bit. 

First, though, I'd want to know what you're doing in Spring 2018. If you're getting a semester of teaching release then, that may be just as good as getting in Fall 2017 and thus there would be little reason to argue.

If you're expected to teach in Spring 2018 as well, then you've got a legitimate beef. Just bring it up politely, ask for some remedy, and remember that it's in your department's interest as well as in yours that you have the time you need to get established in your new department and get your research off to a splendid start.