Read this: Whereas most Christians believe the gibbet on which Jesus was executed was the traditional two-beamed cross, the Jehovah's Witnesses hold the view that a single upright stake was used. The Greek and Latin words used in the earliest Christian writings are ambiguous. The Koine Greek terms used in the New Testament are stauros (σταυρός) and xylon (ξύλον). The latter means wood (a live tree, timber or an object constructed of wood); in earlier forms of Greek, the former term meant an upright stake or pole, but in Koine Greek it was used also to mean a cross. The Latin word crux was also applied to objects other than a cross.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What causes the different accounts of the cross used?
The Greek and Latin words used in the earliest Christian writings are ambiguous