User blog comment:B214/Chapter 383 Surfing Lucy thoughts and analysis/@comment-197.226.178.50-20140509121045/@comment-24264679-20140511023254

You summon based on situation which she summoned well. Loke is her best Combat spirit which is why she summoned him to deal with the approaching dude. Also Sagittarius can deal with a lot of enemies at once with his arrows and is way more reliable than Aquarius. Virgo and lamy is more for comedy on Hiro part also story wise Hiro planned it out a certain way for a reason. He knew Jackal would show up so he needed to hold off until the end to have Lucy summon Aquarius for a more dramatic feeling.