In the first ever episode of Doctor Who, Susan explains that she named their home the Tardis (Time And Relative Dimension In Space). This implies that there are many Time Lord time travelling vessels and the Doctor's Tardis is only one of them.
However, in later series there are multiple references which imply that the Time Lords built (or perhaps grew) the Tardises.
"We're going to build a Tardis" - The Doctor's Wife
"We are the first Sontarans in history to capture a Tardis" - The Poison Sky
This implies that a Tardis is the name of a type of ship.
Is there a discrepancy and if so what's the reason for this change?