I would add that the research advice is that the PPM's are often misleading as to the CS efficacy.
As is repeated here and in the research literature, the CS ion atom groupings called 'particles', size, are the defining characteristic of efficacy (not mentioning the obvious or to re-litigate the ion vs non ion atom efficacy in the matter).
This is why (less than)<25nm @ 10PPM is considered the model for therapeutic use and the smaller the particle the more success we should experience.(no matter the PPM's of larger than 20nm particles).
(and when we consider bacteria size and gram state, the size of the particle can be the one determining factor in generating a therapeutic outcome or not).
Questions here are also, what type of burning, how long does it last, is a genuine Neb being used (which seems likely considering the type of illness involved)?
Since there is no discomfort reported using the product straight, the 'Q's' may be irrelevant and the real 'Q' is why even dilute at all.
If the initial treatment wasn't effective, and subsequent uses were deemed necessary, then one supposition would be as stated. The product is suspect as to it's therapeutic efficacy due to it's particle size, since the staph bacterium is gram positive, (and also has a natural defense against our own immune system), the CS ion would need to enter the bacterium cell and work it's Antibiotic nature there (which it can do well) as opposed to the much easier membrane destruction which occurs with gram negative bacteria, of which the Pseudomonus is (but particle size, not PPMs, being relevant here as well).
One thing that is worth considering is that due to the nature of CF causing thickened fluid to coat the airways, any burning upon using the nebulizer w water would seem to be odd, tho obviously there is a complaint, so as JE mentioned, figuring out the actual culprit may not be that difficult. One 'Q' might be if the water was steam distilled or chemical distilled, and was it structured/revitalized in some way before using (an important but misunderstood aspect to all water ingestion, especially distilled, tho I'm not thinking that this would be related to your burning experience).
But the real question to me is, how is the treatment working on the symptoms of Staph and Pseudomonas. And if a quick reversal or lessening of symptoms isn't noticed, find another(or make another) CS product.