Why is my gas gauge pessimistic?

[Eric Perplies]

I once had to fully drain my fuel tank to have it welded. I pulled
into the shop a few miles after the low fuel light came on. We
drained my tank. When my low fuel light comes on I have 30 gallons of
fuel in the tank. 70 additional gallons (a total of 100 gallons)
caused my gauge to read 3/8 tank. An additional 100 gallons makes my
gauge read about 5/8 full.

Eric Perplies
96' 42' WBWL