Something is out of level. It is either the soffit or the cabinets that you are putting in.
You need to draw a level line somewhere on the wall in between the top of the base cabinets and the underside of the soffit. Then measure up to the soffit in various places to determine if the soffit is out of level and where the lowest point is. To be sure that the line you are drawing IS actually level then you need to flip your level end for end while keeping the same edge facing up to see if the bubble reads the same both ways. If it doesn't you need to adjust your level. Most cheap levels are not accurate.
If you determine that your soffit is actually more or less level then measure down from its lowest point the height of your wall cabinets (front measurement) and draw your actual setting line there. This is the reference line you will now use to set your cabinets. You can tack a board below that line to help you hold your cabinets in place while you screw them to the wall. Make sure to screw the face frames together too. Make sure you allow enough height for your soffit not being level from front to back. usually the area on the soffit where the front face of the cabinets intersect is actually 1/8" - 1/4" lower than the back ceiling/wall inside corner intersection. If this is the case you will need to set your reference line lower. You will always still have some gap at the top. You put a moulding there to cover it up. Also you can screw up through the top face frame into the soffit framing.
If your soffit is badly out of level then you could apply wood over it, basically creating a new, level soffit that could be painted or stained.