interleague plunk rates

Somewhere in this comment there's a question about interleague plunk rates. Here's what I found:
From 1997 to 2005 there have been 2,200 interleague games. In those games, batters have been hit at a rate of about once per 109.4 plate appearances. In total during those 9 seasons, batters have been hit at a rate of one per 109.6 batters, so it would appear that there is no difference in HBP rates in interleague games vs intraleague matchups. But, the overall rate of batters plunked in National League parks during that span is 1 per 111.6 batters, but when the American Leaguers visit, it drops to one plunk per 115.4 batters. In the American League, the overall rate has been 1 per 107.6 batters, but when the National League visits, they roll out the welcome mat and hit 1 of every 104.1 batters. In interleague games played in AL parks, the visiting National League team has gotten drilled once per 94.9 plate appearances while the home American League teams get hit once per 115.5 plate appearances. Perhaps the American League pitchers enjoy showing there enthusiasm for the designated hitter rule in this way. The NL batters get plunked more at NL parks too... once per 113.5 PAs while the AL is visiting, while the American League visitors get hit once per 117.4 PAs.
Overall, through 2005, National League batters had been hit 825 times in interleague play - 366 at home, and 459 times on the road. American League batters got plunked 734 times in those games - 366 times at home, and 368 on the road.
So, it appears that the big difference here is that National League batters seem to get drilled uncommonly often when they visit American League Parks, and Craig Biggio is no exception. Biggio has been plunked 16 times in 137 interleague games (including this season), but only 4 were at home while 12 were at American League parks. He's been hit once per 79.5 plate appearances at home in interleague matchups, and once every 25.3 plate appearance when visiting the American League. I guess either American League pitchers didn't learn very quickly that Biggio was not going to move out of the way of those inside pitches, or Biggio just wants to give the American League fans a chance to see some of his history making plunks during the few times he visits the AL parks. If he'd been at his AL park plunk rate for his entire career, he'd have around 460 HBPs. That'd be great and all, but he would have smashed Hughie Jennings record sometime in 1999 - long before blogging became popular, and before the wealth of baseball data available today could be found freely on the internet.