PCI Express slots on the motherboard can be wider then the number of lanes connected. For example a motherboard can have x8 slot with only x1 lane connected.
On the other hand, you can insert a card using only for ex. 4 lanes to a x16 slot on the motherboard, and they will negotiate to use only those x4 lanes.
How to check from the running system how many lanes are used by the inserted PCIe cards?