Count of occurrences in Power Query

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMgQBJR0lR6VYHQTPCYXnjMJzAfOMQACuD8ZzQuGhqnQF84xBAC4H4zmh8JxReKj63JRiYwE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [SUP_NO = _t, ITEM = _t]),
    #"Grouped Rows" = Table.Group(Source, {"ITEM"}, {{"Count", each Table.RowCount(_), Int64.Type}}),
    NestedJoin = Table.NestedJoin(Source, "ITEM", #"Grouped Rows", "ITEM", "Grouped", JoinKind.LeftOuter),
    #"Expanded Grouped" = Table.ExpandTableColumn(NestedJoin, "Grouped", {"Count"}, {"Count"})
in
    #"Expanded Grouped"

Despite of same results, those two DAX formulae are different. You might want to refre to the explanation in detail,

https://dax.guide/allexcept/

When used as a modifier in CALCULATE or CALCULATETABLE, ALLEXCEPT removes the filters from the expanded table specified in the first argument, keeping only the filters in the columns specified in the following arguments.

When used as a table function, ALLEXCEPT materializes all the unique combinations of the columns in the table specified in the first argument that are not listed in the following arguments. In this case, the result only has the columns of the table and ignores the expanded table.