Hi Therese,
Are you using InfoPath 2007? I was curious about your post, so I tested a drop down control with some 3 digit numbers in it by "tabbing" to the control and typing. It seems that the drop down controls react differently based on the speed you are typing the numbers in. For example, when I typed "1", like you mentioned, it takes me to the first number in the list. When I typed "10" in succession it took me to "101". Likewise, when I typed "183" quickly it took me straight to "183". I tried typing "1", then waited a second and typed "8", but it did not register and stayed in the same spot. Try it out! I hope this helps :)
-Patrick