The problem is that the -U option parser already evaluates
the "verify" flag (see the "HERE!" comment):

      case 'U':
        upd = parse_op(optarg);
        if (upd == NULL) {
          avrdude_message(MSG_INFO, "%s: error parsing update operation '%s'\n",
                  progname, optarg);
          exit(1);
        }
        ladd(updates, upd);

        if (/* HERE! */ verify && upd->op == DEVICE_WRITE) {
          upd = dup_update(upd);
          upd->op = DEVICE_VERIFY;
          ladd(updates, upd);
        }
        break;

Thus, if the -V option has not been present at this point,
it's being ignored ("verify" defaults to 1).

Unfortunately, there's no easy fix for that.