A properly sized cuff is very important for accurate measurements. Purchasing a cuff that is too small for your arm will result in falsely elevated readings. It is better to purchase a slightly larger cuff rather than one that is too small. If you have a large upper arm, purchase a large blood pressure cuff. The pulse is also recorded by all of the electronic machines.
Blood pressure is very variable, which means it can change for a number of reasons.
Blood pressure tends to be lower in the evening and lowest during sleep. 20 mmHg diastolic, depending on how it is measured. The difference is not the fault of the machine, but rather an error in the method of measurement. Use the correct-sized cuff for your arm. Keep your arm at heart level while measuring, preferably by resting your arm on a table in a comfortable position.