Without being able to see any of the Conversation JSON for those calls it is hard to give you a definitive answer, but if they are outbound calls that didn't connect (bad number, endless ringing) then you'll have a startTime and endTime but not a connectedTime. You will only get a connectedTime if the call actually connects.
Note an outbound call that gets an answering machine is likely to show a connectedTime because the call was established on both ends, thus "connected".