You're remembering it wrong, go look at the rulebook, page 87 - (forgot 3-4 is standard skill too, sorry) - it specifically says that if you rolled a stat, you could increase that stat, but if it is at it's max, you can increase another stat instead, no mention of being able to pick a skill instead - actually, nowhere does it say you get to pick a skill, it is all random (quote below for reference)
I'm claiming that in Necromunda you don't get to pick your skills - you do in Blood Bowl and you do in Mordheim. If they want to go with random skills, it's fine by me, I'll play along, but I would rather have some degree of control.
There are seven types of skill and each has its own
separate table: Agility, Combat, Ferocity, Muscle,
Shooting, Stealth and Techno. If you refer to these
tables you’ll see that each offers six different skills.
The type of skill that a fighter can have is restricted by
the gang’s House and whether the fighter is a juve,
ganger, heavy or leader. For example, Goliath juves
can only take Muscle and Ferocity skills, Goliath
gangers can take Muscle, Ferocity and Combat skills
and so on. These restriction are indicated on the Skill
tables on page 88.
On the roll of a 2 or 12 the fighter can ignore the
normal restrictions for his House or type, and select
from any of the Skill tables.
To determine a new skill for a fighter, pick the type of
skill you want from those available, then roll a D6 to
determine which skill has been learned. If you roll a
skill that the fighter already has or that he is not
allowed to take for any reason, you may pick any skill
of that type.
Example: A player rolls a 3 indicating a new skill for
a Goliath juve. Referring to the Skill tables he finds
that he may choose between Muscle or Ferocity
skills. He decides to take a Muscle skill and then
rolls a D6, scoring a 3. Muscle skill number 3 is
‘Crushing Blow’. This is noted against the fighter’s
other details on the gang roster.
An Advance roll of 5-9 will increase one of a fighter’s
characteristics. For example a roll of 7 increases
either Initiative or Leadership. Roll a D6 to see which
of the two characteristic increases applies. To
continue our example, a roll of 1-3 means the fighter
has gained +1 Initiative and a roll of 4-6 indicates he
has gained +1 Leadership.
However, characteristics may not be increased
beyond maximum limits as shown on the
characteristic profile below. If one of the two
characteristics indicated by the advance roll has
already reached its maximum level, you must take the
other. If both have already been taken to their
maximum level, you may choose to increase any
other permitted characteristic by +1 instead.