Varför slutade Lockharts stavning vid armen?

När Lockhart försökte läka Harry's arm och slutade med att ta bort hans ben, varför stavningen bara påverkar hans arm ? Så dåligt som Lockhart är med magi, är det inte omöjligt att föreställa sig att han gjorde Harry till en pool av sopp med den rätta flickan på handleden, så vad slutade stavningen från att lösa Harrys hela skelettet ?

“Stand back,” said Lockhart, who was rolling up his jade-green sleeves.
“No — don’t —” said Harry weakly, but Lockhart was twirling his wand and a second later had directed it straight at Harry’s arm.
A strange and unpleasant sensation started at Harry’s shoulder and spread all the way down to his fingertips. It felt as though his arm was being deflated. He didn’t dare look at what was happening. He had shut his eyes, his face turned away from his arm, but his worst fears were realized as the people above him gasped and Colin Creevey began clicking away madly. His arm didn’t hurt anymore — nor did it feel remotely like an arm.
“Ah,” said Lockhart. “Yes. Well, that can sometimes happen. But the point is, the bones are no longer broken. That’s the thing to bear in mind. So, Harry, just toddle up to the hospital wing — ah, Mr. Weasley, Miss Granger, would you escort him? — and Madam Pomfrey will be able to — er — tidy you up a bit.”
Harry Potter and the Chamber of Secrets, chapter 10