My own opinion, not reflecting those of Marketo or anyone else, it is more how the concept is interpreted than the value itself.
The Enterprise API Design Version 2.1 reference guide refers to Lead Key Value as id:561-HYG-937&token:_mch-marketo.com- 1258067434006-50277. That matches the value stored by _mkt_trk.
561-HYG-937&token:_mch-marketo.com-1374552656411-90718 is an absolute value. It is like referencing Joe Doe as First name and Last Name or just using that value when the variable is implicit.