There are 52 weeks in a year, most eps have between 23-24 eps per season, that run usually between Aug/Sept thru to April/May. In the case of this show there are 23 eps.
Now in saying that there are times when repeats will come into play during the season. Usually around the holidays they may play one new ep (like last weeks "Still Life") then play repeats.
Usually the reason for this is, because alot of people are shopping, having relatives coming in, basic holiday things.
Not to mention the fact that the actors film so many eps in advance, and then get time off to either amp the show, to do other projects, or what have you.
Then you have the networks, who have the holiday shows, they tend to not want to put up a show (even highly rated) against a "holiday special".
So do they have a true reason for repeating the rest of the month? probably, Will they share with us that reason? Most likely not. Or is it the possiblity to build anticipation maybe for ratings, or just the new eps, could be.
No one knows for sure unless they actually come out will full throttle explaination on it. Hope this helps some.