It's true. I've noticed that some jalapeno peppers are hotter than others. Some can be quite mild, while others can be downright eye watering. I prefer somewhere in the middle, but this is something that cannot be avoided. Get the full answer here.
While the jalapeno heat is contained in the seeds and veins (or placenta) of the pepper, jalapeno pepper heat levels do vary depending on many factors such as age of the pepper, how many overall seeds are within the pepper, when and where it was grown (including climate and soil conditions of that location and cultivation), not to mention weather and amounts of rainfall.
That said, a jalapeno pepper can vary in heat level from between 2,500 and 8,000 Scoville units. Want to learn more about Scoville units? Click Here.
If you find that a pepper is too hot, you can always reduce the heat by removing the seeds and veins (innards), but it is difficult to make them hotter without dumping habanero powder over them, which I've done in the past. Also, the red jalapeno peppers tend to be sweeter and not quite as hot, so you can choose those. Jalapenos will turn red the longer they are on the vine and eventually fall off.
So, next time you pick your jalapeno peppers from the grocery store, or any other peppers for that matter, you can expect some heat variety.