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This is commonly misunderstood, the performance difference between the interface and the device.
The interface has dedicated hardware that can transmit or receive the L2 frame at wire-speed. I.e. an Ethernet gig interface can transmit or receive an individual frame at gig.
Often an interface might have additional hardware, that it can send or receive multiple frames, back-to-back, at full wire-speed too.
However, on software based routers, the main CPU normally needs to "process" the packets, and depending on how fast a processor it has, it can only process packets at some maximum rate. You'll normally see this listed as packets-per-second (PPS) for different packet sizes. (NB: in olden times, often rate was quoted for maximum sized packets, now it's generally quoted for minimum sized packets.)
Often the perforamance of a small router cannot substain an interface continously running at wire speed.
Some thing else to understand, processing a packet is somewhat independent of its size. So you'll also find a software based router's capability to pass packets at a certain "bandwidth" decreases as the frame/packet size decreases and conversely increases as the frame/packet size increases. (NB: as noted earlier, this is why bandwidth throughput used to be quoted for maximum size packets, bandwidth throughput performance looked much, much better on marketing literature.)
If you're wondering why have interfaces that support much higher "bandwidths" than the router itself can, there's are several reasons. First, individual frames/packets have less serialization delay. Second, vendors probably treat some hardware components as commodities, i.e. it's less expensive to use the same interface hardware on multiple routers. Third, it looks better in the marketing literature (i.e. two gig interfaces rather than two 10 Mbps interfaces).
