Because the Products Carousel uses a different QWeb template than the main shop grid, and that template automatically appends the combination name when the product has attributes — even if those attributes do NOT generate variants.

What happens is:

1. You added an attribute (even with "No Variants").

2. Odoo tries to compute the product display name using:

   product.display_name
   

3. When attributes are present, Odoo builds a name like:

   “Product Name (Internal Reference)”

   because the system thinks it is a variant-like product.

4. The main shop page (website_sale.products) cleans the title and hides internal references.

   BUT the carousel block uses a simpler snippet template, which does not hide internal references — so it shows the full product display_name.

That’s why:

Carousel shows internal reference

Shop page hides it

Remove internal reference from display in carousel (QWeb override)

Create an inherited view for:

website_sale.products_item

Then replace product name rendering:

<t t-name="your_module.remove_internal_ref" inherit_id="website_sale.products_item">
    <xpath expr="//div[@class='product_name']" position="replace">
        <div class="product_name">
            <t t-esc="product.name"/>
        </div>
    </xpath>
</t>

This makes the carousel use only the product name — not the computed display_name.