The simple answer is, I don't think they really DO think about such things, not much or for long...it strains their faith, which leads to guilt that they had doubts. The standard answers then become either "God lets it appear this way to test our faith" or "The Devil is tempting/deceiving you". Part of the problem being that JWs are discouraged from higher education, where they might actually learn enough science to make the lightbulb appear over their heads. There are lots of examples of this, when I was growing up in the WWCG (which is quite similar to JWs), a few fun questions to ask older people were:
Did you know that even at lightspeed, when you look at the sun what you're seeing is actually light that has been on it's way 8 minutes ago? (Many will want to appear knowledgable, or may have even heard this factoid before, so will answer "yes") Then you hit them- Well, did you also know that it takes roughly 2 1/2 million years before the light from Andromeda reaches us? (This will stump most of them, but if you can get them to accept the premise, you've got them in a corner already, they just don't know it.) So, if it took 2 1/2 million years for that light to get here...what does that mean to the creation theory, where the earth is only 6,000 years old? Most will hem and haw and fade back into pat answers or the ever present "god works in mysterious ways" / "we can't begin to understand the god-tastic universe". Just a few... a very few, may begin to think.
Diamonds was another favorite of mine in this category...most women have at least their wedding ring if married, or a favorite pair of earrings, whatever- people are basically familiar with diamonds. So you ask them, "Did you know it takes millions of years to form first coal, then diamonds, from carbon?" You see where I'm going with this, and many times they would too... standard answers were often something on the lines of "God must have created them already made".
Trying to talk too much logic with people mislead into controlled paths such as the JWs or WWCG is much like trying to win an arguement with a kid over Superman. "Superman is ultimately strong, he can break anything". "Superman is ultimately tough, he can take anything". Oh yeah kid? So what if big bad Superman punched himself? *blank stare*