from 737NG-FTD-29-01003
"Retraction of the landing gear causes a noticeable drop in system A hydraulic reservoir level. Starting with the system at 100% before flight, retraction of the landing gear causes roughly 20% drop in indicated system A reservoir quantity.
Thermal contraction during cruise causes an additional drop in reservoir level. Operators have reported 5 to 10% further drop in system A reservoir level after some time spent in cruise, and Boeing's calculations indicate that thermal contraction after prolonged cruise at low temperatures could cause as much as 20% drop.
Similar changes of system B reservoir level can occur. Boeing's calculations indicate that thermal contraction could cause as much as 20% level drop. Leading edge extension just prior to landing removes an additional 20% from the reservoir."
Given that Boeing recommend max fill levels of 90%, its not unusual to see A system down to 70% in flight.