The Skill Charts in a lot of the Rules make no distinction between CLAN vs IS nor stipulating that CLAN regular is actually IS veteran.
This is backed up in part by the ALPHA STRIKE COMMANDER'S EDITION page 29 with the SKILL RATING. There is no CLAN or IS column
. . . they do not because . . .
TW p271-273, for randomly determining skills based on forces' experience rating.
Specifically, once you have the experience rating of the force (Regular, Veteran, Elite, etc), you roll for the actual piloting/gunnery skills. Clan mechwarriors get a +2 to the roll, meaning they are always bumped to the next higher roll (the table goes 1-2, 2-3, 4-5, 6-7, etc).
So, WHY have a new chart when you insert a single sentence that takes up less page space that says give Clan mechwarriors a +2 to the roll results? Additionally, Alpha Strike makes no distinction of Clan weapons having longer ranges (aka, its a different detail level), so I am not sure why you are trying to use a book for a different system to determine TW to determine a TW stat.
No discrepancy exists, its been this way since the Clans were introduced and IMO that +1 with original TW nckestrel cites is someone mixing up +1 level with +2 to roll, both can mean the same thing if put in correctly but it slipped through proofreading to use parts of both phrases.
As for what makes a unit Elite . . . there are some intangibles that we have a hard time translating over to a direct TT setting, and some will only have a impact when you get into a campaign setting. For instance I have seen references to a study that determined a infantry soldier from a elite information would accomplish 2 or 3 more missions than a regular infantryman due to the selective nature of the elite formation. But how do you represent that on table top? You make them shoot better, but that is a really clumsy way IMO to give that advantage- but its also fast when you are just wanting to drop dice & minis on the table. Like I said for my campaigns, I am thinking of using SPAs for any results that make elite Clan- or even veteran Clan pilots- too good.
IMO Elite units, besides being better gunners & pilots, are going to exhibit-
-SPAs for individual mechwarriors/aeropilots/BA troopers/vehicle crews
-Unit abilities, not sure we have any on the regulars since I do not have the FMs with me but I think Regular rated units might be limited to fighting environment abilities (like Arcturan Guards get arctic environment advantages) or like some of the Bear formations get a bonus to random rolls on unit weights. But elite units are going to IMO have more and combine some like . . . forcing the initiative, off map movement, jungle environment training.
-morale & forced withdraw benefits
-commander bonus/abilities, like 'When Star Captain Super is on the board, his trinary gets +2 to initiative' or extra edge
-greater access to alternate munitions or Omni-pod options, which is not something generally discussed
-greater call on strategic assets like jumpships/artillery/airstrikes/SpecOps/MASH
-first in line for repairs/replacements
But all that stuff is complicated to put on the table, so we get the simple 'Elite Clan formations are 1/2' to abstractly give those advantages.