Below is an explanation as to how the CDC calculates the average incubation period. My question is references step 3 be

[answerhappygod]

Below is an explanation as to how the CDC calculates the
average incubation period. My question is references step 3
below:
Count Back the Average Incubation Period
The average incubation period for Shiga toxin-producing E. coli
(STEC) is 4 days. Now, count back 4 days from the peak. That date
is December 6.
Shiga Toxin-producing E. coli Cases
by Date of Onset, Port Yourtown, December 2011
The following steps will allow you to determine the most
likely period of exposure for a point source
outbreak based on the epi curve.
My question is when looking at step 3, how did they come
to arrive at getting 2 days for the minimum incubation period (see
step 3 below):
Step 3: Count Back the Minimum Incubation Period
Identify the earliest case in the outbreak and count back the
minimum incubation period, which is two days. That date is December
7.
Shiga Toxin-producing E. coli Cases
by Date of Onset, Port Yourtown, December 2011