Certain consonants are frequently used interchangeably (e.g., b and p, d and t). Thus, Shepsel may be recorded as Shebsel; Mordka becomes Mortka, etc. See the chart "Phonetically Interchangeable Consonants" in Shmuel Gorr's Jewish Personal Names: Their Origin, Derivation, and Diminutive Forms, page xi; or the Daitch-Mokotoff Soundex System chart for a good list of these types of substitutions.
Vowels are particularly interchangeable in various languages and transliteration systems. The Yiddish name for "Yitzchak" (Isaac) might be written on a ship manifest in a bewildering number of ways — as Icek, Itzig, Hayzjk, Ayzig, Izyk, Eisik, Jck, Iccok, Yitzik, Jczek, Idzk, etc.