Dear Sayani,
As you know pre-grant opposition takes place before the grant of the grant of the patent, and the language in Rule 55 focuses of term applicant because the application for patent is not yet granted to the applicant.
When the patent application is granted the now the applicant is referred as patentee under Section 2(1)(p) – patentee means the person for the time being entered on the register as the grantee or proprietor of the patent.
Language in Rule 58 focuses on the instances after the grant.
Hope it is clear now.