Steve,
That is perfectly normal. The ground phase number will change all the time when sweeping or holding the coil up in the air. Ignore the ground phase number unless you are pumping the coil to balance or check your balance. It is only accurate when you pump the coil over the ground.
The FE304 bar graph is also only accurate when you pump the coil, however, it will also change over targets, especially if you make multiple passes over them. For instance, if you normally have 2 bars showing, and sweep over a horseshoe a couple of times you will see the number of bars increase. If your ground is mellow enough you can use this to help id the iron targets. Since most of my ground already maxes out the FE graph I am unable to use this feature. But you may be able to.
Rusty washers are a good target for FE identification as are steel bottle caps. Both may give high tone and high id numbers but both will spike the FE meter if your ground FE reading is low to start with.
Ground phase readings over targets in mild ground can be somewhat useful as well. The phase reading will also measure conductivity if there is not a lot of FE mineralization in the ground. You would need to pinpoint the target, then pump the coil over the target to see it. You can see this in air by bobbing a couple of different objects over the coil. It will work in low mineral ground (one or two bar) I don't mention this much just because my ground conditions negate being able to use these features.
Good luck,
Mike