The answer is that I and V are similar in their biochemical properties. By the way I see some V-I substitutions as well as the I-V in query vs subject and also some others (e.g. B-D, L-M). These are all real and natural variations in the sequence.
If you look at the table here, you will see that Valine and Isoleucine are both hydrophobic and similar in structure. If you delve deeper into the numbers, you will see further that they have similar charges, similar sizes, and are aliphatic. This means that they are likely to conserve protein structure in the folded form. Valine is less energy expensive in synthesis for the cell, so perhaps this plays into it a bit, but as this is a virus that is hijacking the cell processes I have no real idea if the energetic cost comes into it.
BLAST, as mentioned in the comments, returns a certain number (usually 50) of the most similar sequences to the query. You can adjust the number and specificity limits during your setting of the search parameters. If you do something like blast the spike reference sequence from Wuhan-Hu-1 (YP_009724390.1) against your sequence (put yours in query, and the Wuhan-Hu-1 one in the subject or vice-versa), you will see a number of substitutions, indels etc, none of which are I-V. From this you can deduce that your findings of I-V and V-I are simply a matter of sampling based on the most similar sequences.
I don't know the algorithm for BLAST (or anything else for that matter), but it may well take similarity of biochemical properties into account when searching for most similar sequences. Given that there are millions of submitted sequences for the spike of SARS-CoV-2, there are likely to be many thousands of highly similar sequences in the database.