A number of things.
1) dirty/contaminated pcb, leads, input jacks, test probes.
2) bad input protection components (PTC, MOVs, fusible resistor, spark gap).
3) weak, low battery. The Fluke 70 runs an amazing 2000 (two thousand) hours on a 9V battery, but a low or weak battery can cause funny readings.
Try a fresh battery first. If that doesn't work, then I suggest a thorough IPA cleaning of all the components listed in #1 and then retest. Let the pcb dry completely by running a fan or letting it dry for 24 hours.
For #2, the components are listed in the Fluke user/service manual and you can test all the components in circuit and compare them to the values listed in the manual.