Do you take BP medication? If so, is there any link between when you take it and the change in pressures?
Also, unless you are a night-shift worker, the morning pressure is probably taken after 8+ hours of rest, whereas the evening one has a full day's activity behind it.
I've only got an 'intelligent' layman's interest, so consult your doc. if worried, but it doesn't sound too bad a difference to me.