What is your basis for that? That doesn't fit my observation at all, and I'd go so far as to say it's a bizarre statement.
Middle class is middle class. Generally it means people are in fairly secure employment, or sometimes self employed, and they normally have regular access to things like good dental care and medications and so on, and some kinds of investments, even if it's only a pension. These days they will generally have some kind of post secondary education.
As for differentiating within the middle classes - it's not a hard line. LMC are more likely to have good jobs in trades, some kinds of para-professional positions. They will likely have access to a pension of something that functions as a pension, but not usually a lot of other investments - if they live in a place where they can afford a house that may well be their major investment. (A lot of these people would, a few generations ago, have been working class, but they changed in the workforce provide a higher income and they do have investment income now which is a big difference from the past.)
Upper middle class would be the very highest earners, the "professional middle classes" often they will be people whose professions are regulated by professional organizations. They might be considered "rich" by some, but their incomes, while high, aren't usually enough to put them at the level where they are making money work for them, they are working for money. And while they may be able to spend a lot of money of a few things , a home, cars, private education, travel - they probably can't spend like that on everything. Many doctors, lawyers, certain people in business, sometimes certain people in the arts or even the top of the charitable sector can fall into this category.
In the middle you find a lot of people like civil servants, teachers, less successful lawyers, small business owners, university professors, and successful tradesmen with their own businesses or the ones who can ask for a very high wage.