Probably this explains it why shared key worked:
IPv4 Tunnel Network : The suggested default in the GUI of 10.0.8.0/24 is sufficient, but any random unused network inside of the RFC1918 space is recommended. For site-to-site shared key, only a /30 is used, not a /24, even if /24 is specified.
Taken from: https://doc.pfsense.org/index.php/OpenVPN_Site_To_Site