PPP is making a comparison with its own previous poll in mid-May, showing the same numerical margin (Akin 45, McCaskill 44). It is more appropriate to make a comparison with polls conducted after the August 6th primary, since after a primary a nominee’s support firms up considerably.
Akin was +3% (median, n=3) before August 6th, and +11% (n=1) afterward — an 8-point bounce. So if he is unchanged since May, then he must have lost all this gain. An 8-point loss in one day is an enormous drop.