It's a property of the (spoken) language, not just of writing, and it's called syncope. It is a very common process across languages: a weak vowel before the final consonant of a word often disappears before an ending or suffix.
Latin magister 'master', plural magistri
Russian у́гол (ugol) 'corner', plural углы́ (ugly)
Georgian მეგობარი (megobari) 'friend', plural მეგობრები (megobrebi) [the -i is the nominative suffix in either case: when the plural suffix -eb is added, the -a- disappears.]
It is usually not universal: in a given language, usually some words show it and others don't.