Why did the astronaut want Catherine to run the numbers?

Question

In the movie Hidden Figures, where the background exceptional work of Catherine helped the struggling USA to keep up with Soviet Union, a scene arrives - towards the end of the movie, where the astronaut wants the protagonist to compute his landing coordinates.

Why?

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My understanding: the IBM electrical computer is said to give inaccurate landing coordinates between just two runs of the same program.

In USA's endeavor to catch up with the world's first ever human in space by the Russians, they are sending an astronaut (I think Glenn), with Friendship 7, into space. However, the team leader of Catherine realizes that something is off with the landing coordinates, since they differ from yesterday's calculation. The about-to-be second human into space then specifically asks for Catherine, in order to assign her the task of manually doing the calculations and give the coordinates, with as much precision as possible ("I could use every digit you have for me", he says).

However, floating point arithmetic is known to be deterministic, meaning that the same program (code) should give the same results amongst different runs, as long as the input is the same.

The paragraph above makes me think that I am missing something big time.

Answer 1

It is based on a real event:

"Get the girl to check the numbers."

With those seven words, spoken by astronaut John Glenn before he became the first American to orbit Earth in 1962, Katherine Johnson's role in history changed.

A "human computer" assigned to NASA's Flight Research Division at Langley Research Center in Hampton, Virginia, Johnson was the African-American, then-44-year-old "girl" who was the subject of the astronaut's directive. As such, she set about double checking the trajectory calculated by her electronic counterpart: room-size IBM 7090 computers at Goddard Space Flight Center in Greenbelt, Maryland.

"If she says the numbers are good, I am ready to go," said Glenn, according to Margot Lee Shetterly, citing Johnson, in her book "Hidden Figures" (William Morrow, 2016) which served as the basis for the 20th Century Fox movie by the same title opening wide in U.S. theaters on Friday (Jan. 6).

Don't forget that Glenn et al had for decades relied on "human calculators", and that computers were a newfangled invention that were operated by possibly flawed human beings.