Ok here goes!
For the sake of this, let's throw out weight, because weight equals a range of flex numbers. Say a 14' 150lbs could be a 21.0 to 20.0 flex (not accurate, I am not sure of the real numbers without my chart at work) so a 21.0, 20.9,20.8.....20.0 are all a 14' 150. Got it?
How do you get a flex number?
A specific distance for each length is set up and measured on a pole and the pole is hung on these points, then a specific weight is placed near the middle and the ammount of bend is measured, then the weight taken off, and measured again, the difference in these numbers, in cm, is your flex number. So if you hang a 14' pole, on these points, measure with and without weight, and the difference between these is 20.5, then the pole is a 14' 150, because it falls in the range of 21.0-20.0. Now, this is a bit simplistic, but the concept is solid. The stiffer the pole, the less bend with the weight, the lower the flex number, and stiffer the pole. So if you measured a 14' pole, and it comes out to a 19.9, then it is no longer a 150, it would fall into the next range of 155. Gettin' it?
Part of the trick.
If you have 2 14'150's one may be soft, say a 21.0, and another may be stiff, 20.0, so they are not the same pole. You in theory almost have a 14'145, and a 14'155, although they both say 150, and are 150's according to the range given.
Another part of the trick.
The span (or distance between points where the pole is hung) changes to accomodate the length of pole. Now not all manufacturers use the same span either. Some may use a slightly larger or smaller span, or measure the span closer to the top or bottom of the pole, or use a slightly different wieght, all slightly changing the flex, and possibly weight of the pole. So in essence a 14'150, 20.0 spirit may not equal a pacer 14'-150 20.0 essx, or the pacer 14'150 20.0 pacer, because they measure them slightly differently. ALL MANUFACTURERS ARE EXTREMELY ACCURATE AND CONSISTENT WHEN THEY MEASURE POLES! They just do it lightly different. I would not have any trouble telling someone to move from a 150 pacer to a 155 spirit or essx if he or she needed the next pole, they are that close.
At Bell Athletics we combat this problem by measuring all our poles ourselves to ensure that all things remain constant. That means we can ensure the numbers are always accurate same span, weight, ect. no matter what brand. Take for instance one of my poles, a 5.00m (16'-4") 15.9 (a 195 I think) carbon. Now from the factory it says flex 15.9, we measured it to be a 16.4 on our scale (because our span, weight, ect. is different than those at the gill factory, no better or worse, just different). But the pole smaller than it in my bag is a Gill number 16.4, our flex number was a 16.8. So I know no matter what color the pole was, that it is the next bigger pole for me. So if I blow through my (bell number) 16.8, I grab the 16.4 and all is well, it is .4 bigger, and I should jump higher.
Flex numbers are a more accurate measure than weight. They are a bit more tricky, but it may help you to figure out some problems in your series of poles. If you have weights that are close, it is possible to have flex numbers that are still quite a distance apart. As an expieriment you might want to check your bag, and check the differences in flex numbers. How much different in flex numbers is it from one pole to another. For my poles they go, as I go up poles, .6,.6,.5,.4,.4,.5,.4,.4,.5. Now these are small differences, I have several 180's,185's,190's,and 195's all 5.00m long. Differences for everyone else should be larger, my bag is overkill for most. Bigger jumps are OK. In your bag did you find any big jumps, or small jumps. That may explain why some poles feel way bigger, or just slightly bigger than others.
There is more to it than this of course, but this is a good overview.
Hope this helps.