Avira, their current intermarriage rate is close to 80%. The younger generations are much less likely to be Jewish than the older generations. Even 30ish years ago their intermarriage rate was considerably higher than what you’re basing your math on.
Additionally, they’ve been performing false conversions for well over 100 years already. I do believe they perform such “conversions” at a much larger figure than you’re counting. 1,000 in 2013 sounds like HUC alone, not counting the far greater numbers by individual Reform clergymen across the country.
The 20% reporting to Pew a non-Jewish mother would only count those surveyed that are aware their mother is a gentile and willing to say so. Most are unaware.