Hello! It’s probably just because he doesn’t care. Jonny purchased his main FV-300H, which he used from late-1995 until 2011, after his original BOSS FV-100 was stolen in October 1995. Given his general attitude, I wouldn’t be surprised if he got an FV-300H rather than an FV-300L by chance. Perhaps whichever guitar store he ran to after the gear theft had an FV-300H in stock, and it served Jonny’s purposes. You couldn’t just buy an exact replacement on eBay back then, especially if you’re in the middle of a tour.
A photo of Jonny’s pedalboard in the early 90′s, showing his original BOSS FV-100 Volume Pedal. The white pedal next to the FV-100 is the Marshall Shredmaster clone which Plank built for him, and which he used from 1993 until it broke in 1994 or 1995.
From a technical point-of-view, low-impedance volume pedals are better suited to where Jonny places them in his signal path, but placing a high-impedance volume in the same spot doesn’t make very much of a difference. Low-impedance volume pedals exhibit the optimal sweep and tone when placed after a buffer. The sweep of a high-impedance volume pedal is altered by placing it after a buffer, but it has no effect on the guitar’s tone and still works perfectly fine. By contrast, placing a low-impedance directly after a guitar’s output will cause some significant loss in treble frequencies and signal strength.
Given that Jonny used a high-impedance volume pedal after buffered effects for decades, he may have become accustomed to its more abrupt sweep. He may even have he tried both an FV-300H and an FV-300L back in 1995 and at the time preferred the sweep of the high-impedance model when placed after his Shredmaster. However, Jonny has mixed low and high impedance volume pedals since 2009, so in all likelihood he just doesn’t care about the difference in sweep between the two impedances. If he has any feelings about the differences between volume pedals, it may have to do more with the physical feel of the pedals than with their internal electronics.
A photo of Jonny’s pedalboard at the band’s Austin City Limits taping in 2012. That was the only year that Jonny’s guitar pedalboard featured two volume pedals: the second was added to the end of his clean signal path exclusively in order to perform the song Feral (changes to Jonny’s guitar part have since made the pedal unnecessary). The board features two high-impedance volume pedal: a standard Ernie Ball VP JR. (250K) and BOSS FV-300H. Earlier in 2012, Jonny used the low-impedance FV-500L in place of the FV-300H, but for this performance the FV-500L was paired with one of his keyboards. Jonny had also used the FV-500L in place of the Ernie Ball in 2009.