It seems the idea of forecasting boffo B.O. for horror films has frightened studios lately.Universal’s “Mama,” which bowed over Martin Luther King Jr. weekend, earned $32.1 million in four days; a few weeks earlier, Universal chiller “Texas Chainsaw 3D” scored in the high teens in its opening frame. Both considerably overperformed their projections. While managing box office expectations is always a prime motive for studios, the early modest tracking for both horror films had almost every pundit fooled. In the case of “Mama,” however, the film’s PG-13 rating — the only one among the weekend’s wide-release films — was an asset, especially among distaff teens. “There hasn’t been a PG-13 film targeted at young girls in a long time,” says Universal domestic distribution prexy Nikki Rocco. “Mama” also drew a slightly larger-than-expected male turnout, 39%, which likely had a negative effect on the holiday’s other wide entries, Fox’s “Broken City” and Lionsgate’s “The Last Stand.” Still, “Mama” scored the majority of its coin from women under 25. January is the usual home-away-from-home for horror films outside the early-October corridor leading up to Halloween. The genre tends to do well at the beginning of each new year after the glut of sophisticated year-end awards fodder. That said, the overperformances of “Texas Chainsaw” and “Mama” has been a product of a generally robust market lately: Every weekend so far this year has outpaced its comparable 2012 frame.