Disclaimer: this is highly speculative.
I believe they mean farming seasons, and that these seasons are not related to the orbital period.
According to Wookieepedia, moisture farms were collecting water to raise underground crops. It is unlikely that such crops would care about the solar cycle. (Now is the time for another disclaimer: I do not know much about biology.) For example, in what seems to be the closest real-world example, fungiculture, seasons last 60-100 days.
Furthermore, Tatooine orbits around two suns. I assume that the definition of a year is based on a full rotation around the pair of suns, but then the position of the suns will be roughly independent from the time of the year. Hence, predicting the weather on Tatooine based on the time of the year could prove difficult. (Daily weather forecasts are easy though: "It will not rain".)
Additionally, while not a proof, I don't remember hearing or reading anything about "summer", "winter" or "rain season" (???) on Tatooine. (Most Star Wars planets seem to have this kind of constant weather, though.)
So, without any physical indication of the time of the year, maybe the farmers are using the rhythm of their crops instead, hence their use of "season" instead of "year".
In this context, I take the word "semester" to mean "half a season".
Another reason for thinking in "seasons" instead of "years" is that the variety of solar/planetary cycles in a large galaxy makes our earthly time differences look like a funny joke. According to the EU (but still the most likely solution canon-wise), the Star Wars galaxy was using Coruscanti years for that purpose. Non-Coruscanti could then have reserved the word "year" for galactic years.