Attendended Transfer - Why so many open channels?

When doing an attended transfer via Asterisk (typically *2) I’ve noticed that several channels are opened and this causes severe lag on my old system. The scenario is as follows: SIP extension 7335 dials SCCP extension 6000. 6000 transfers (attended) 7335 into a MeetMe conference. Then 6000 hangs up and dials back into the same MeetMe conference. The conference is now unusable because of latency. A blind transfer does not produce this. This is the resulting channels:

asterisk1*CLI> show channels Channel Location State Application(Data) SCCP/6000-00000009 86000@from-internal: Up MeetMe(86000|asMp) SIP/7335-644f s@macro-dial:10 Up Bridged Call(Local/86000@from- Zap/pseudo-166815924 s@from-pstn:1 Rsrvd (None) Local/86000@from-int 86000@from-internal: Up MeetMe(86000|asMp) Local/86000@from-int s@from-internal:1 Up Transferred Call(SIP/7335-644f 5 active channels 2 active calls
Here’s what a blind transfer looks like for the same scenario:

asterisk1*CLI> show channels Channel Location State Application(Data) SCCP/6000-0000000b 86000@from-internal: Up MeetMe(86000|asMp) Zap/pseudo-115101211 s@from-pstn:1 Rsrvd (None) SIP/7335-3a8d 86000@from-internal: Up MeetMe(86000|sMp) 3 active channels 2 active calls
Why 2 extra channels for the attended transfer? After all, the extension that does the transfer DOES hang up.

(Lag & open channels exist even when only using SIP extensions, it’s not a SCCP issue)

Asterisk 1.2.5