"decrease in reception strength of about -20 dBm, "
This is sloppy and the author can't be right. The sentence may also be a double negative.
20dB is a horrific reduction in signal. They can't have meant that. -20dB IS a reduction. decreasing by -20dB means increasing. -20 dBm (note the m) is an absolute measurement, not a delta. The sentence is like saying reducing by sea-level when they meant reducing TO sea-level.
The correct wording might have been "decrease in...