If it's 5dBi, why does it say "2dBi Detachable Omni-directional Dual Band Antenna 2dBi detachable omni-directional antenna provides high performance and increased coverage for your wireless network." ?

It is dual band, just like it says in the first sentence of the description.

Questions & Answers: OURLINK 600Mbps mini 802.11ac Dual Band 2.4G/5G Wireless Network Adapter USB Wi-Fi Dongle Adapter with 5dBi Antenna LAN Card

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Item#: 9SIACMW5833298
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OURLINK 600Mbps mini 802.11ac Dual Band 2.4G/5G Wireless Network Adapter USB Wi-Fi Dongle Adapter with 5dBi Antenna LAN Card

Wireless speed up to 600Mbps
Selectable Dual Band
Detachable high gain External Antenna
Compatibility: Win10, 8.1 , 7, XP, Mac OS X 10.6-10.12.4 Sierra

$11^.98 –