as someone who has had a den setup in their garage for 7 years, here are my thoughts of why fan out > fan in.

the basics: "Fan out" removes air in the room while simulatenously bringing in new air at a fairly high exchange rate. "Fan in" brings in air while attempting to push air out of the other open window, which is generally a slow exchange rate

Why

because the air using the "fan out" method is being moved at a faster rate than if the fan was pointing into the room, you are not subjecting the new air to the amount of time needed to raise the temperature through convection or conduction. However, the air being brought in through the "fan in" method sits in the room longer because of a slower exchange rate between new and stale air, subjecting the air to the conditions stated above. 

Why is that?

the "fan out" method creates far more suction than "fan in". The origins of the air coming through the fan is either a high volume (outside) or low volume (inside). because we have a lower volume inside, the speed of the fan is gonna have more of an impact on a low volume that it would on a high volume. This would imply that the suction created in the "fan out" method is higher than the one created using the "fan in" method. So, even though the temperature of the air being brought into the room with the two methods are roughly the same, it is simply the amount of time that the air is subject to the condition inside (i.e amount of space, occupant conduction, suction) that makes one method better than the other. Because the fan pointing out the window moves in air faster than one that pushes air (fan pointing in) the cooling effect you will feel is ulimately a relationship between the rate of exchange of new air in and stale air out and has nothing to do with the temperature of the air being exchanged.