I see. It's using the whole template, rather than the series.
To be precise: the "Replacement Template" is the template which the formation will try to pull replacement elements to match (from formations marked for use as replacements). If your infantry battalion has the "Training Battalion" as its replacement template, it will try to pull replacement elements from other formations to match the OOB of a Training Battalion.
The "Replacement Template" is
not the type of formation from which reinforcements will be drawn. Understandable confusion.
The Unit Series system defines types of elements, not formations, and is really intended to be used to replace units of the same type which might have multiple versions. For example, your first Infantry Rifle might be researched with racial attack and armor of 8, and later you may research Infantry Rifle Mk II with attack/armor 10 and place it in the same series. This allows replacements to be pulled from the older mark if the newer mark is not available, or allows formations composed of older units to be replenished with newer models. This aside, the important thing is that while Series and Templates work together they are two different systems not to be confused.