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The Zyxell states that it has 225 Mbps SPI firewall throughput, not 100. The UTM throughput is 30Mpbs. The 100Mbps limit may be due to an intermediary 100BaseT port or a poorly graded cable.
As for why you are only seeing 70Mbps, while vendors are known to inflate their numbers, a 155Mbps difference seems a bit odd. I suggest you double check to make sure that all features that require any kind of processing power are disabled.
Look at your firewall stats for retransmissions, CRC errors and other errors. Ultimately, apart from finding someone with your exact model who is also in a position to be able to saturate their line for a test, I suggest you take this issue up directly with Zyxel tech support.