What did a cappella originally mean, and why were some Renaissance a cappella works not truly unaccompanied?

The term a cappella is derived from the Italian phrase meaning "in the church style" or "in chapel or choir style". While it is understood today as a performance by unaccompanied voices, the term originally referred to sacred choral music.12

Some Renaissance a cappella works were not truly unaccompanied because, although composers did not write independent instrumental parts, historical evidence indicates that an organ or other instruments were used to "double" some or several of the vocal lines. Scholars previously assumed these works were performed solely by voices because no separate instrumental parts were recorded in the manuscripts, but it is now clear that instrumental support was a standard practice in the performance of such sacred music.12