This is there custom. The word "manner" in the Greek is "ethos" and pertains to that which is a usual or customary manner of behavior. It may also be defined as a habit. Thayer explains that many of these customs have their original prescription from the law.
When you see this word in other sections of the New Testament, it is mostly identified as an accepted custom of a group of people. It was customary for the Jews to assemble in Jerusalem for the Passover feast (Luke 2:42). It was a custom for the Jews to be circumcised, therefore they struggled with accepting the uncircumcised Gentiles (Acts 15:1). Jesus was buried according to the accepted manner of the Jews (John 19:40).
Perhaps the most significant word in Hebrews 10:25 is the word "some."This word means only certain ones. Some is not all, and yet some is also more than none. The implication of these two ideas together is that there are a few who have made it their custom to be different than the rest. The problem is that this custom is not according to a commandment or law, but rather is disobedient to the command to assemble when the saints have gathered together.
Habits are hard to break. I have come to understand that there is no sermon, no Scripture, and no approach that will break the manner of some. But my prayer is that each one will love the Lord and His church enough to assemble at every opportunity with the body of Christ.
"I was glad when they said to me, 'Let us go to the house of the Lord.'" ~ Psalm 122:1