Nobody who intends to wait for a warning shot bothers to stealthily unholster their pistol underneath the table. Also, a gun pointed at your chest across said table is warning aplenty. And remember, warning shots are a tool of people who have no desire to shoot the person they've drawn on but may be required to do so if the target continues to act threateningly. Greedo stopped Solo on the way out of the bar and forced him back to the table. He had the upper hand, but wasn't smart enough to ensure he kept it by either disarming Han or forcing him to keep his hands atop the table.
Holy crap, if you weren't so frickin' pathetic you'd be hilarious.