I have been a teacher (Pre-school - middle school) for 15 years. I agree with what Adrian said about learning in stages, but I also wanted to add that kids understand more than they can produce. (For example, when you are learning a new language, you can understand what is being said before you can say it yourself or read before you write).
Also, it seems that some kids have a type of "stage fright" or a performance "block" when you are asking them direct questions.
My son will walk around the house counting up to 20, but when I ask him to count, he can't get past 2 or 3 without my help. It is like he has it "hidden" in his sub-conscience (sp?) and it just comes out when he is very relaxed and not really trying. I have noticed this as very common for kids of many ages. An example for older kids would be when a kid swears they can't add fractions, but they can double their chocolate chip cookie recipe in a snap!
I personally would need more information about your child before I would say that a Speech Eval was neccessary, but on the other hand, it couldn't hurt. Weather you find out that everything looks great or you get some early help, it is a win-win. Plus, it is provided free of charge.
Another thing that is important to remember is that there is no hard fast rule about exaclty when children should learn this or that. Everyone has their own pace and own special gifts. I notice this all the time with my child and his same age friends.
I also will never forget the wonderful example of this that I experienced early in my teaching days. I had 2 boys in my preschool. They were both about 2 1/2 years and their birthdays were less than one month apart. One was very small, still wearing diapers, and walking with that cute baby waddle. The other boy was tall and thin, had been potty trained for a while and was one of the fastest runners in our school, even against 4 year olds! The other big difference between them was, when I handed them crayons and paper, the tall boy would try to eat the crayons or break them, they would never touch the paper. The other boy who was still in diapers, could draw an amazing portrait of his family, where you could recognize who was who, with glasses and correct hair colors and heights! My point is they were both amazing kids who were very advanced for their ages, they both just had different gifts! I lost track of the taller boy, but the smaller one will graduate HS soon. He is an honor student, plays the saxaphone and studies German, probably could have predicted that huh? I bet the other boy is some type of star athelet.
Anyway, good luck with your son, it sounds like you have a great learning adventure before you!