Adam Clarke on Exodus 25:15

The staves - shall not be taken from it - Because it should ever be considered as in readiness to be removed, God not having told them at what hour he should command them to strike their tents. If the staves were never to be taken out, how can it be said, as in Num 4:6, that when the camp should set forward, they should put in the staves thereof, which intimates that when they encamped, they took out the staves, which appears to be contrary to what is here said? To reconcile these two places, it has been supposed, with great show of probability, that besides the staves which passed through the rings of the ark, and by which it was carried, there were two other staves or poles in the form of a bier or handbarrow, on which the ark was laid in order to be transported in their journeyings, when it and its own staves, still in their rings, had been wrapped up in the covering of what is called badgers' skins and blue cloth. The staves of the ark itself, which might be considered as its handles simply to lift it by, were never taken out of their rings; but the staves or poles which served as a bier were taken from under it when they encamped.