Why did Allah Almighty use the word "Astajib" (أستجب - I respond) instead of "Ujib" (أجيب - I answer) in the noble verse: "Call upon Me; I will respond to you"?

The form "istaf'ala" often denotes a request, but it can also come to mean something other than a request, such as "astajib" (I respond) and "falyastajeeboo li" (let them respond to Me) meaning "to answer," and "istawqada" meaning "to kindle," and "istazallahum" meaning "to cause them to stumble," and "istahwathu" meaning "it cast him into a pit," and "la yastanqidhoohu" meaning "they cannot rescue him," and "istarhaboohum" meaning "they terrified them," and "ista'marakum" meaning "He settled you," and "fastabshiroo" meaning "rejoice."