Ten minutes spent reading down an old index will give you at least ten reasons why not. Even surnames were sometimes abbreviated in the days of pen and quill and expensive paper. There's a good possibility that the name is only indexed under the abbreviated spelling a much safer policy, when the abbreviation is not marked with a period or superscripted letters. Most of the time they didn't try, but instead, entered the abbreviated form as a separate listing. For example, Stephen, Steph., and Steph'n would be separate listings.
Old records have other quirks as well.
Military officers were frequently mentioned without the use of their given names. Captain Allen, for example. In an index, titles may be at the beginning of the Allens, before Allen, Adam. And while you'd still find this electronically, you wouldn't find it if you didn't know your ancestor was a military officer and you looked him up only under his baptismal name, or under Captain Robert Allen. Indexes may suggest many more spelling variations than you ever dreamed of.