Odd progressive forms of Verbs of Motion

In http://www.alphadictionary.com I read:
"The verbs denoting motion are a special case in Russian. Instead of the usual aspectual pair of forms, verbs of motion have three aspectual forms: perfective, progressive imperfective, and iterative imperfective. Don't fret! There is a system to help you remember these forms. The perfective is always formed by simply adding the prefix по- to the progressive form. The iterative imperfective usually ends on и- or ай- [it refers to the theme] ". Everything clear. Then shows two lists, one of intransitive verbs of motion and the other transitive verbs of motion. The headings are like this:

(Perfective)
Progressive Iterative Odd progressive forms Gloss

(по)ид- ходи- (по)шёл, шла. кли go on foot
(по)бежа- бегай- (по)бегу, бежишь, бегут run
aso.
And the same for transitive verbs of motion.
However I have clear (as far as it is possible) this matter of verbs of motion I can't understand well this heading of "Odd progressive forms", specially the word "odd". In my dictionary are a lot of meanings for this adjective but I don't know which of them will fit better. Which other heading will you put instead of "odd progressive forms?
I can't get well the columns, but I suppose you will understand Thank you.

I think they are calling the progressive forms “odd” because their conjugations are irregular. From the point of view of someone who considers only читать and говорить type verbs to be regular, all those progressive forms have irregularities in either their present or past tense forms.

Thanks. As for "odd" and "regular" verbs in Russian. In every grammar says: "In Russian there are only two conjugations", and I suppose that after saying this the authors will laugh :lol: . Of course, if in the first lesson the author says:" ... and the first conjugation has a lot of varieties; and every variety has a lot of exceptions. By the way, the second conjugation, as well", the student will not reach the second lesson. As for me and however it all I find the study of Russian very addictive.

I agree, radomir. I have studied eleven languages, and Russian still fascinates more than any of the others. (Although I think Polish is more difficult, which theoretically should make it more interesting.)