I think that it is still surprising considering how clean the A130 measured. Same tweeter I assume? It is puzzling. I also wouldn't necessarily assume it can't be heard on this speaker. When you start to get to inexpensive speakers like this you always have to wonder about running changes and quality control issues as well.
I don't think the distortion is coming from the tweeter. I think it's from the woofer - a larger, different woofer than the one in the Stage A130.
The Stage A130 uses a 5 1/4" woofer.
The Stage A180 uses two 6 1/2" woofers.
If I may make a generalization, a smaller woofer behaves better in its upper range, while a larger woofer behaves better in its lower range.
The Stage A180 woofers appear to have some breakup around 1.5kHz that isn't present on the smaller woofer on the Stage A130. From the plot of the nearfield responses, it appears that the bottom woofer has a notch filter around 1.5kHz to kill that breakup region. But the upper woofer, which needs to play a little higher, doesn't have that notch filter. The breakup is there in all its ugliness. I think that's where the distortion is coming from - the upper woofer playing a little higher than it really should.
Here is the Stage A130 driver response:
Notice how the woofer response is fairly clean in the 1-2kHz region. It does show some breakup around 2.5kHz (higher in frequency than the breakup of the larger woofer), but this is above the crossover and is being filtered somewhat by the lowpass filter on the woofer.
Now look at the driver responses for the Stage A180:
The lower woofer has a deep notch around 1.2kHz. The upper woofer has a bump there. That bump is where the distortion occurs. The fact that the lower woofer has a deep notch filter on it, not just a lowpass filter with a lower cutoff frequency, tells me that JBL was aware of the breakup of these woofers in this region.
The Stage A170 model, which uses the same drivers as the A130 (two of the 5 1/4" woofers), should be cleaner in this region - at the expense of less bass/higher distortion in the bass compared to this Stage A180 model.
EDIT NOTES: I made some additional statements for clarity.