An SMS message sends up to 160 characters. With the automatic footer included, it allows for 114 additional characters. 

MMS sends two versions, a 500-character MMS message and a 160 character non-MMS message. With the automatic footer and content link included in the MMS message, it allows for 454 additional MMS characters, and 84 additional non-MMS characters. 

Why is the character limit for a non-MMS message shorter than a standard SMS message?

Any MMS message that is created will also include a non-MMS version of the message for the carriers that do not support MMS.  The character limit of the non-MMS message is adjusted by 30 characters to allow for a system-generated URL that provides access to the MMS content (i.e. graphic, video, audio file). This is why the default non-MMS message allows for 84 additional characters instead of 114 characters.