Why is Mahatma Gandhi sometimes referred to as Gandhiji?

It is simply a variant spelling of the same name.

http://www.indiachildnames.com/mobile/name.aspx?name=Gandhiji

The -ji is an honorific - it shows someone is respected. A bit like -san in Japanese. https://en.wikipedia.org/wiki/-ji Apart from the title system in UK and such places, we don't have an equivalent in the Western world.

Mohandas Karamchand Gandhi was the original birth name of Gandhi.In fact Mahatma means a saint or a person with a great soul.Because of Gandhi's non violent policies and his tolerant attitude towards everyone he earned that title.Ji in Hindi means a word of respect.If some one is older in age the word Aap or Ji is attached to the person's name when addressing him.Mahatma Gandhi is also known as Bapu which means father.Hence he is also referred to as Bapuji.