Why expected_value contain negative number?

[ZoeM2016]

I use my data and the code is as follows
results = detect_ts(c,
max_anoms=0.2,longterm=False,
direction='neg', only_last='day',e_value=True)
My original data did not contain negative numbers,but some expected_value are negative.why?What is the formula for calculating expected_value?
10 1980-09-27 18:00:00 0 0.128816
11 1980-09-27 19:00:00 0 -2.732376
12 1980-09-27 20:00:00 0 -3.102446,