Well, there can be and mostly are many reasons for this. Some are individuals, some are programs some are the big boys trading on avg.
Regarding expensive depends how you’re set up. There are many different ways to go here, too many to go into. I.e. for me flat fee unlimited trades or lots trade. Buy 100, 500 1000 trades etc per year Limits are in play however, i.e. 5000 or 25000 share trade max and so on. How much business you do in other areas and how big your portfolio is can also get you some good cost per trade. Especially over the last few years.
Bottom line not a factor if you’re a trader. Don’t worry about the pennies, worry about the dollars. Many don’t
Anyway not for everyone , want to repeat this ,NOT for everyone, most traders make their money off the people who shouldn't be. This IMO but, its good advice if you don't already. Not the time for it anyway.