My meagre understanding is that the "seven times for bhikkhus" is in the Vinaya, which is based on an actual case example during the Buddha's time.
Now, for the bhikkhunis, there was no such historical case example for their Vinaya. However, it has become the assumed and default understanding that this means they only have one opportunity. Even though there is apparently no actual explicit statement either way for the bhikkhunis from the Teacher Himself.
(Though I'm very open to evidence to the contrary from the learned.)
My recently moved Blog
, containing some of my writings on the Buddha Dhamma, as well as a number of translations from classical Buddhist texts and modern authors, liturgy, etc.: Huifeng's Prajnacara Blog