Butterworth bandpass - higher roll-off than expected
When the ratio of the bandwidth to center frequency is low, the quality factor goes high and the response no longer has nice, smooth, constant rolloff outside the edges. In your case, I see ~200Hz~300Hz badwidth, with a ~1kHz center frequency, which makes the ratio less than 1. It's also what Andy says, about the log axis, but here, log or lin, the rolloff is initially faster as opposed to further away from the edges. Here's an IIR Butterworth, f0=48kHz, fc=1kHz, and bandwidth stepped according to the following table: 100, 200, 500, 1k, 2k, 5k, 10k [Hz]. Note that the linearity of the rolloff is only reliable further away from the edges the lower the BW/fc ratio (log axis):



For comparison, here's the same sweep on linear axis: