Measuring the voltage between (positive/negative) terminals of PV system and ground
With no ground connection, the intrinsic capacitance of the solar cell causes it to charge up. It reaches it's open circuit voltage across the + and - terminals. You got that part right.

What you possibly didn't think of is that it's actually a giant network of capacitors, rather than just a single one.

Also remember that in materials "open circuit" doesn't quite exist, and even insulators leak a tiny amount of current, so each one of those capacitors is being simultaneously being charged by the sun and leaking current to discharge the capacitors too. If the solar panel is uniformly constructed, you would expect half it's normal open circuit voltage on the +ve terminal, half on the -ve terminal (in the other direction), and your virtual ground to be somewhere in the middle of the panel.

Your multi meter is working by draining a little current - and equating the voltage to the current drained. Your reading is effectively moving that "virtual ground" you had before in the middle of your panel towards your measurement point because you are discharging some of the capacitors (and thus allowing others in the middle to charge).