Net-metering allows the annual retail value of production to offset the annual retail cost of usage. Each individual kWh produced or used is valued in accordance with when it was produced or used. So 2 kWh produced while they're worth 20 cents each can offset 4 kWh when they're worth 10 cents each. Solar PV produces more in the summer and in the day than in the winter and in the night. Meanwhile, many rate schedules have higher energy rates in the summer and in the day than in the winter and in the night. This means solar PV tends to produce the most while energy is also the priciest, yielding more bill savings per kWh produced.
Tiered pricing can also play a role. If the first 1000 kWh used costs 10 cents/kWh and the next 1000 kWh used costs 20 cents/kWh, then 2000 kWh of usage would cost $300. If solar offset 1000 kWh, or 50% of the usage, it'd offset $200, or 66% of the bill (assuming no other charges).