Why is it so complicated to multiply some numbers

Tranga/Chapter.cs

@@ -13,16 +13,21 @@ public struct Chapter
     public string? chapterNumber { get; }
     public string url { get; }
     public string fileName { get; }
+    public string sortNumber { get; }
 
     public Chapter(string? name, string? volumeNumber, string? chapterNumber, string url)
     {
         this.name = name;
-        this.volumeNumber = volumeNumber;
+        this.volumeNumber = volumeNumber is { Length: > 0 } ? volumeNumber : "1";
         this.chapterNumber = chapterNumber;
         this.url = url;
         string chapterName = string.Concat((name ?? "").Split(Path.GetInvalidFileNameChars()));
-        double multiplied = Convert.ToDouble(chapterNumber, new NumberFormatInfo() { NumberDecimalSeparator = "." }) *
+        NumberFormatInfo nfi = new NumberFormatInfo()
-                            Convert.ToInt32(volumeNumber);
+        {
-        this.fileName = $"{chapterName} - V{volumeNumber}C{chapterNumber} - {multiplied.ToString(new NumberFormatInfo() { NumberDecimalSeparator = "." })}";
+            NumberDecimalSeparator = "."
+        };
+        sortNumber = decimal.Round(Convert.ToDecimal(this.volumeNumber) * Convert.ToDecimal(this.chapterNumber, nfi), 1)
+            .ToString(nfi);
+        this.fileName = $"{chapterName} - V{volumeNumber}C{chapterNumber} - {sortNumber}";
     }
 }

Tranga/Connector.cs

@@ -73,7 +73,9 @@ public abstract class Connector
         XElement comicInfo = new XElement("ComicInfo",
             new XElement("Tags", string.Join(',',publication.tags)),
             new XElement("LanguageISO", publication.originalLanguage),
-            new XElement("Title", chapter.name)
+            new XElement("Title", chapter.name),
+            new XElement("Volume", chapter.volumeNumber),
+            new XElement("Number", chapter.sortNumber)
         );
         return comicInfo.ToString();
     }