Unspecified Altitude

Why is the filed altitude for flights taking off late always say “UNSPECIFIED?”

flightaware.com/live/flight/ASH7 … /KCOS/KORD
flightaware.com/live/flight/UAL6 … /KLGA/KORD
flightaware.com/live/flight/EGF4 … /KDTW/KORD
flightaware.com/live/flight/AAL1 … /KMCI/KORD

Could be the flight plan had timed out, but the controller still had the paper strip so instead of having the crew call dispatch to file a whole new flight plan he/she just re-entered the basics such as call sign, type a/c and route into the ATC computer. The requested altitude would then be taken care of just before or just after takeoff and would not make it into the filed flight plan that we see.
The other time you see “unspecified” is if the flight is a VFR flight receiving VFR advisories.