Words and phrases have multiple, context-dependent meanings. That's just how natural language works.
"a fraction of ..." is commonly used in informal and/or non-technical contexts to mean what would be expressed in a more technical context as "a positive, proper fraction of ..."
"a fraction of ..." is commonly used in informal and/or non-technical contexts to mean what would be expressed in a more technical context as "a positive, proper fraction of ..."