That's because the Kickstarter included all of Dirty pair - the entire series, OVAs, and Features in an exclusive art box-within-a-box for a pledge tier of $250. You could also get the art box with the $135 tier, but only with the series itself, and you would then have to buy the other sets to fill the box either during the slacker-backer or at retail from Rightstuf 6 months after the Kickstater shipped out to the backers.
And, 3301 backers is actually a substantial number for an anime Kickstarter - I've pledged in 15 anime Kickstarters since 2013, and this one has the most backers and highest total dollars except for Under the Dog (the only one creating a new anime from scratch). Gunsmith Cats also had more backers. Only Nozomi's Aria Kickstarter had a higher average pledge, by just $3.52. For my own future reference, I took the time to create a list of all the anime Kickstarters I participated in, showing the Number of Backers, Total Pledge $$, and Average Pledge $$ for each one:
2711, $215,433, $79.46 - Time of EVE: The Movie on Blu-ray
2175, $154,014, $70.81 - Bubblegum Crisis Ultimate Edition Blu-Ray Set
1903, $107,153, $56.31 - Mai Mai Miracle
12157, $878,028, $72.22 - Under the Dog
1509, $102,869, $68.17 - Otaku no Video Otaking Edition Subtitled Anime Blu-Ray
2058, $318,784, $154.90 - The Vision of Escaflowne: A New HD Dub for the Classic Anime
1752, $135,589, $77.39 - Riding Bean High Octane Edition Anime Blu-ray
1646, $224,546, $136.42 - Skip Beat! Official North American Release
2648, $595,676, $224.95 - Let's Dub ALL of the ARIA Anime for Blu-ray!
4249, $350,879, $82.58 - Gunsmith Cats Explosive Edition
1615, $253,834, $157.17 - Let's Dub ALL OF Emma A Victorian Romance Anime TV Series
1451, $154,031, $106.16 - Megazone 23 Omega Edition
1418, $208,567, $147.09 - The Girl from the Other Side feature-length animation
2295, $184,906, $80.57 - Metal Skin Panic: MADOX-01 Unobtanium Edition
3301, $730,930, $221.43 - Let's Dub the Dirty Pair TV Series!