Distinct count based on previous dates and grouped by other variables

change the column [all] in the expressione where

    #"Added Custom 1" = List.Transform(#"Added Custom"[all],

to [rollingQ]

    #"Added Custom 1" = List.Transform(#"Added Custom"[rollingQ],

but, please PLEASE, be carefull to the space in the text values. For instance Blue is not "Blue" but "Blue " and this cause problem.

I  don't have now enough time to analyze and understand the real reason for these problems, but, for safety, find and remove the extra spaces

#########edited#############

use this code to try

let
 
   s1 = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("nZTNbsIwEIRfpcqZSKU3jqCqhwh6IBcqxCGA+0MtQEhpVZ6+Voyn9o4Xou4hsfAnz2ZnzHJZDMvnw1c5HBWDovHlVhPbmjv3rovVIEHWXbnF3Gzdcyb3/454MdYevt1iemEezcYzm0tFOguCtl1BqJb7xlcsFJqpmn35cO9+eA0VKU0l9NYVhBYEZD6plgzGEmSoFzG4PircyntXGX80GWrjA5WzKNMNOfRk1pJJLQKAXsghILtQGYcAffpii2bNSVOqJYI8SYeYSB3CfhRbtmh8PHHm0rkAQeJoLkB4/iQknOZO9KFckeFjkDnlayj6kT0/mdClx4DR0pQBaGxgqBdqJQ4cpUlXom4wlX80U7X72+kHpKYfhG40EDX9TIj/p7B/Nf2g9PRXrb0hZHsJ2d5C+jUDol8zINo1y5yh65x9CWb1Cw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Date = _t, ID = _t, Var1 = _t, Var2 = _t]),
    s2=Table.ReplaceValue(s1,"Blue ","Blue",Replacer.ReplaceText,{"Var1"}),
    #"Changed Type" = Table.TransformColumnTypes(s2,{{"Date", type date}},"en-US"),
    #"Filtered Rows" = Table.SelectRows(#"Changed Type", each [Date] <> null and [Date] <> ""),
    #"Grouped Rows" = Table.Group(#"Filtered Rows", {"Date"}, {"all", each _},GroupKind.Local,(x,y)=>Number.From(y[Date]>=Date.AddMonths(x[Date],1))),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "rollingQ", each Table.Combine(List.LastN(List.FirstN(#"Grouped Rows"[all], List.PositionOf(#"Grouped Rows"[all],[all])),3))),


    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcsopTVXSUQpWitWBc3yROT7InAgILyg1Ba4HwvZFYiOrgWqITM3JyS+H64FzfVG5aIpBmmMB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t]),
    #"Changed Type 1" = Table.TransformColumnTypes(Source,{{"Column1", type text}, {"Column2", type text}}),

    #"Added Custom 1" = List.Transform(#"Added Custom"[rollingQ], (t)=> Table.AddColumn(#"Changed Type 1", "dist", each try Table.RowCount(Table.Distinct(Table.SelectRows(t, (r)=>  r[Var1]=[Column1] and r[Var2]=[Column2] ),"ID" ))otherwise 0)),

    q1=Table.FromColumns({#"Added Custom"[Date], #"Added Custom 1"}),
    #"Expanded Column2" = Table.ExpandTableColumn(q1, "Column2", {"Column1", "Column2", "dist"}, {"Column2.Column1", "Column2.Column2", "Column2.dist"}),
    #"Added Custom 2" = Table.AddColumn(#"Expanded Column2", "mmm", each Date.MonthName([Column1],"it-IT")),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom 2",{"Column1"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Columns",{{"mmm", "Column1"}}),
    #"Pivoted Column" = Table.Pivot(Table.TransformColumnTypes(#"Renamed Columns", {{"Column1", type text}}, "it-IT"), List.Distinct(Table.TransformColumnTypes(#"Renamed Columns", {{"Column1", type text}}, "it-IT")[Column1]), "Column1", "Column2.dist")
in
    #"Pivoted Column"

The problem about space ending Blue was that I hand writed an auxiliary table for the colours and sizes and, of course, I couldn't realize there was a space after Blue in the table you linked