Because Charlie had gone ahead he didn't see Theo's wobble, and by the time he reached the head of the stairs and turned, ahead of the slow progress of the platform, Theo was stable and nearly caught up with him.
Charlie's arm had automatically reached out to catch Theo as he stumbled, even though he wasn't quite close enough to be effective, but it was late and he'd been heading home to sleep so his reaction times were a little slowed and Theo had already managed to catch himself.
"Thanks, Dominic Chambers did all the work on that and the area downstairs that blocks out a lot of sound," he said. He hesitated for a moment after holding open the first door for Theo. "Would you like an arm?" he offered. Clearly Theo was struggling but he'd flinched from touch earlier and Charlie didn't want to push. "We've gotta get through that next door and across the floor to get to my office for the Floo."