the first 24 h of bleeding

You are on call, and a 10-year old who had a tonsillectomy 5 days ago arrives from the emergency department with post-tonsillectomy hemorrhage for planned surgical intervention. Describe the time frame and causes of post-tonsillectomy bleeding. 

There are two periods of post-tonsillectomy bleeding. The first is known as primary hemorrhage and occurs within the first 24 h of bleeding. It is suspected to be the result of inadequate surgical hemostasis. The second time frame is referred to as secondary bleeding, and most commonly occurs 5–12 days postoperatively as a result of premature eschar separation. Rates of primary bleeding range from 0.2 to 2.2% and secondary bleeding rates range from 0.1 to 3%

你正在值班，急诊科送来一位5天前做过扁桃体切除术的10岁患儿，患儿有扁桃体切除术后出血，计划手术止血。请叙述扁桃体切除术后出血的时间段和原因。

扁桃体切除术后有两个出血期。第一个时间段称为原发性出血，发生在出血的前24小时内，常被认为是手术止血不充分所致。第二个时间段称为继发性出血，最常发生在术后5天~12天，原因是焦痂过早分离。原发性出血发生率为0.2%~2.2%，继发性出血发生率为0.1%~3%[37]。

老师，the first 24 h of bleeding出血的前24小时？我知道他想表达：发生在手术后的24小时内

为什么这样写，有什么玄机么？谢谢