Pressure differentials between eyes?

 In the last couple weeks, my right eye has measured higher pressures than my left on 'most' days. The differential is a bit higher in the mornings and can be about 4-5pts (i.e, typical average L=15, R=20). When my pressures are overall less, the difference is about 2-3pts (though not all the time).  However, looking back at several months of records this seems to be more of a recent occurence. For those practicing self tonometry, have you noticed a bilateral shift in pressures over a several day period? I cannot determine any variable that might explain this.