German tax refund to UK Pension Scheme - 100%?

My company pension fund has received a refund of German tax withheld from dividends.

The Germans had withheld tax at 21.1% from the gross dividends. I reclaimed all of this, because my company pension fund is exempt from tax. I have successfully reclaimed all withholding tax deducted from other countries' dividends, e.g. Holland, Switzerland and Eire and I've got the grey hairs to show for it. The Germans have refunded only 6.1% of the tax deducted, i.e. that in excess of the treaty rate. Has anybody experience of this? Have they simply failed to notice that this is a UK pension scheme or are they correct? The accompanying letter is in German which unfortunately I don't understand. If they should have refunded 100%, can you suggest a convincing reference/ article I can hit them with?