Curious compression reading

[Graham Barker]

When I purchased my 1937 Ruby with standard high compression head 18 months ago, compression readings were:
No 1 to 3 cylinders 100-102 psi
No 4 cylinder 80 psi.
I ground the valves on No 4 cylinder and compression improved to 100 psi.
All good.
Today, with 200 miles clocked up since the valve grind on No 4, the pressure on No 4 has risen to 110 psi!
Compression on cylinders 1 to 3 remains at 100-102 psi!
What could explain the increase in pressure on only No 4 cylinder ? Could it be carbon buildup?
I would be interested in reading any experience relating to this.
P.S. All compression testing was done on cold engine with throttle fully open.

[AustinWood]

Increase explained by grinding the valves on No 4. Maybe grind 1-3 as well?

[Graham Barker]

The increased pressure in No4 cylinder from 100 psi to 110 psi has occurred over 200 miles since the valve grind. Is it common to find that pressure increases with driving after a valve grind?