mention that max PoE out current is 500mA and max power consumption of single unit to be 6.5W (at 24V supply voltage that translates to 271mA).
So to be on safe side, supplied power adapter is capable of powering almost
3 units at the same time (probably it could power 3 units as probability of all 3 drawing full power at the same time is less than certainty). And, again to be on safe side, only one unit can be powered through master's PoE out interface.
Any other interpretation might lead to problems.
BTW, I'd expect to see full power draw not at startup but rather during full WiFi load: around 1W emitted max power through antenna (that's not average power which cheap power meters might display, average is less due to Tx/Rx time sharing) and PA is not perfect (some losses there). Add power to process packets that are about to get radiated. All that on top of usual routing/bridging/firewalling ... when booting it's just CPU running at full speed and some initialization of wireless card.