Guesstimate - An estimation challenge

Suppose I ask you to test category 'Books' on www.flipkart.com.

Let's take the task of testing 'Send email' feature. As a project lead, I might say it takes 5 man days to test this feature. Suppose that a tester needs to pick this task and complete it in 5 days time.

In above scenario, 5 days time is a rough estimate. An estimate is a guideline, not a deadline. The project lead would have arrived at number 5 due to one of the two reasons below:
1. He just picked a random number out of the time available for completing testing
2. He knew the product in and out and made this guess

What factors the project lead may not have considered about the feature and the tester while estimating?
a. Time required to explore and learn the feature by the tester
b. Time required for ideation and test design for that feature
c. Test data avaialability for sending emails. For eg, Anti-virus testing - Procuring Eicar files as attachments while sending emails
d.