由于它的行为必须如此。

Because it must behave that way.

一个UpdatePanel的异步回还是仅仅是：一个回发。如果多个回发被允许并行发生，他们的新ViewStates常常出彼此同步。然后，第二天回发会抛出一个ViewState的验证错误和/或第三回传完成后的回发修改页面的状态，一会丢失。

An UpdatePanel's asynchronous postback is still just that: a postback. If multiple postbacks were allowed to occur in parallel, their new ViewStates would often be out of sync with each other. Then, the very next postback would throw a ViewState validation error and/or one of the postbacks' modifications to the Page's state would be lost after the third postback completed.

在另一方面，页方法和web服务调用不是由WebForms的视图状态或页面生命周期担保。因此，许多那些可能发生在平行的浏览器的同时请求限制允许（通常为4-8，但在一些浏览器如IE6仅2）。

On the other hand, page method and web service calls are not encumbered by the WebForms ViewState or page life cycle. So, as many of those may occur in parallel as the browser's simultaneous request limit allows (usually 4-8, but only 2 in some browsers like IE6).